Caesar very much was the junior partner. Gelzer makes that quite clear, he even describes Caesar in those words (errr, in German, but whatever). Prior to his consulship in 59 he had done very little of particular merit--in fact, his political career had moved pretty slowly, since he had had to spend several years on the run from Sulla. He was, however, a man with a serious reputation by the elections of 60. Caesar's boldness and political daring were quite well-known, as his early career had been spent flouting Sulla's authority, he had prosecuted on behalf of the people (even provincials!) several leading magistrates, had vehemently supported several agrarian laws, had been prominent in supporting the Transpadanes in their bid for enfranchisement, had led the charge in the prosecution of Rabirius, and most recently as praetor-elect had delivered his famous speech against Cicero's decision to execute the Catilinarians without trial (it should be noted that he was also among the most famous orators of his day--after his prosecution of Dolabella Suetonius quotes Cicero as saying that there was no orator in the city greater than Caesar). All of this paled in comparison to Pompey and Crassus, though, who had served as consuls in 70--for Pompey that had even been (arguably illegally) his first senatorial magistracy! Caesar had close relations with both Crassus and Pompey. He had been a strong supporter of Crassus for some time, and Caesar had helped Pompey get his extraordinary magistracy against the pirates.

What Caesar offered Crassus and Pompey was the ability to get the legislation which they had both, particularly Pompey, been striving for for years. Pompey had been trying to get his Eastern Settlement passed and his land grants for his veterans approved since his return from Asia, and Crassus had publican contracts that needed to be revised. They had been blocked by their opponents for years now, and their refusal to play nice with each other didn't help. Caesar offered them the ability to both get what they wanted, and succeeded in reconciling them. Caesar very much was the junior partner--Gelzer noted that until the Conference at Luca, Pompey and Crassus never met each other once in discussion about the triumvirate, it was always Caesar acting as messenger. Caesar's opponents for the consulship were rather weak, and one of them, Lucceius, was his political ally, so Caesar's chances of being elected and getting the opportunity to propose this legislation were pretty good--the bribes that Crassus and Pompey threw around made it all but inevitable. The decemvirate in particular was of great importance to all three--Crassus seems to have supported Rullus' version of it, proposed back in 63, and Pompey desperately needed to settle his veterans. Caesar's actions during his consulship were quite bold, and although we don't know if Pompey and Crassus were aware of how he planned to get around the normal workings of the state, I don't see how they couldn't have. Caesar ignored Bibulus' protests and forced him to lock himself in his home all year. He had Cato sent off to jail when he wouldn't shut the hell up (admittedly Cato was an ass). He transferred Clodius to the plebeian order to allow him to stand for the tribunate and attack Cicero when Cicero proved less willing to sit still than he had earlier seemed. He threatened to prosecute Lucullus for crimes committed during the Mithridatic Wars when Lucullus wouldn't shut the hell up. I mean, the man got stuff done

Caesar was able to get all this done with only the request that Pompey and Crassus assist him in getting his consulship and later that Pompey help block Cato's attempts to prevent him from taking a province, instead being given a proconsulship over the trees and pastures of Italy. It's important to realize that the triumvirate, such that it really existed, wasn't really what we'd call a political alliance. It was a sort of agreement, really intended to be one-time, in which Pompey got his legislation, Crassus got his, and Caesar got his province. Beyond that they agreed, as Suetonius put it, "ne quid ageretur in re publica, quod displicuisset ulli e tribus," "that anything should not be done in the state [by them], which would be unsatisfactory to any of the three." That was really it--beyond those specific laws they wanted passed they basically just agreed to not interfere with each other. Not much of an alliance, although the Conference at Luca altered the way the triumvirate worked significantly, by extending Caesar's proconsulship and granting Pompey and Crassus another consulship--Caesar's standing among the triumvirs was by then also much higher.

EDIT: Also, by the time Pompey came back from Asia he was way wealthier than Crassus. To put this into perspective, Crassus' fortune was evaluated by Pliny as being 200 million sesterces, and Plutarch gives it as 7100 talents, roughly the same amount, on the eve of his Parthian expedition. Plutarch reports that when Cassius captured Rhodes he exacted from private citizens in tribute of gold and silver eight thousand talents, and then fined the city another five hundred (for some reason the 1918 Loeb erroneously translates ὀκτακισχίλια as 800, which is wrong it means 8000, see Liddell & Scott). This was done without sacking the city--Arrian says that when Cassius took the city he issued orders that any man who resorted to violence and plunder was to be executed, and collected the money in an organized fashion from the temples and by threatening private citizens with execution. Rhodes was well past its heyday, yet it could yield such money, although through extortion. The rest of Asia, says Arrian, was to yield tribute to Brutus and Cassius for the next ten years--many of these cities were vastly wealthier than Rhodes had been. Pompey had controlled all of this, and before the ratification of the Eastern Settlement every cent of the tribute paid by every city from Cilicia to Jerusalem was going straight into his pocket. Indeed, Badian showed in his study on the Republican provinces that there were plenty of provincial governors whose wealth rivaled Crassus'--we know of quite a lot of extraordinarily expensive real estate and art deals, they're quite common throughout this period. Sulla's daughter sold a villa for 10 million sesterces, and famously Cicero's prized possession, his house on the Palatine that Clodius destroyed when he was exiled, cost him 3.5 million sesterces, and he had something like a dozen villas scattered throughout Italy--and as Shackleton Bailey pointed out, Cicero was not considered particularly wealthy among the senatorial class, though he was certainly no pauper. Crassus wasn't there because he was rich. Crassus was there because he was the leader of a particular political group that opposed Pompey's