Ok, I'm seeing a lot of speculation here (including some weird stuff about yogurt, for which there isn't even a word in Latin) and no reference or discussion to any of the texts, ancient or modern. Food is hardly in my specific purview, but I know the ancient world and more importantly I know the texts; lest this madness continue any further I will be forced to take the field.

If we define a meal as the consumption of food and drink at a (roughly) set time of the day then we can find references in the texts to three or even four meals quite easily. As early as Plautus the Romans were taking a ientaculum (the word is a diminutive derived from "hungry" or "fasting"). In Plautus' time this was probably still being followed by cena, then vesperna, an evening meal. Sometime before the late Republic this changed. Lewis & Short cite the epitome of Paulus:

cena apud antiquos dicebatur quod nunc est prandium. Vesperna, quam nunc cenam appellamus

Among the ancients what now is prandium was called cena. And vesperna what we now call cena.

Plautus makes reference to a merenda (mentioned by Paulus and others), which was some kind of light afternoon meal. But the point remains that to cultured society--what Romans might have termed "civilized"--there was one major meal of the day, cena. To the Roman mind, influenced by Greek (post-Homeric, as in Homer there are three meals) ideas of commensality, especially prized the evening meal, which grew in size among the educated, wealthy class and took on a decidedly sympotic character: what the symposium was (culturally, at least) to the Greeks, so the cena was to the Romans. The earlier meals--ientaculum and, by the late Republic, prandium--were light affairs, and while like the Greeks the Romans reclined at table in the company of their peers these earlier meals were not even necessarily eaten sitting down, much less reclining. If one now redefines our definition of a meal to an occasion of (at least potential) commensality, then the earlier meals are relegated to little more than snacks, which while not equivalent to the Romans' perception is possibly a little closer than thinking of them as the same thing as the cena.

The Appendix Vergiliana preserves for us a poem mentioning moretum, a sort of paste containing garlic, vinegar, oil, and other ingredients that apparently rural farmers ate sometimes--the poem's subject is the farmer preparing his breakfast. This leads us into the greater question of "ordinary" eating habits, which is where the texts begin to fail us somewhat. The diet and eating habits of the rural plebs, as far as we can tell from references in the texts and bucolic poetry and stuff like that, probably followed a relatively old-fashioned--obviously frugal--habit, eating whatever was in season from the produce of their farms and so forth. Of the urban plebs probably we can speak a little more definitely (or I can at least). The dwellings of the urban plebs, the horrifically cramped and dangerous insulae of the city, contained as a rule no hearths. Therefore, the urban plebs by necessity ate out-of-doors (there was little light in their windowless cells anyway) and did not cook for themselves. Sometimes they attended the dinners of their wealthy patrons (for all of this see Garnsey's triple books Famine and Food Supply in the Graeco-Roman World; Cities, Peasants and Food in Classical Antiquity; and Food and Society in Classical Antiquity), but most often they survived on little but grain alone. The role of grain as the staple of the ancient diet cannot be understated (and there's more than one book or article on it: see "σιτομετρεία: The Role of Grain as a Staple Food in Classical Antiquity"). The Roman urban plebs in particular survived pretty much wholly on grain. The grain market was so crucial that minor fluctuations in the price of grain, caused either by suppliers hoarding their grain (and trading in grain futures) or by actual naturally bad harvests, could cause mass hunger in the city. Apart from the state-subsidized grain (which was never enough to feed a family), free after 58 but greatly cut back in scope by Augustus, who turned it into a hereditary privilege, the price of grain needed to be stabilized, as it was in the early Principate. Since the urban plebs had no hearths they relied on outside ovens to bake their bread, either at communal ovens or by paying bakers to bake their grain. More usually, however, they ate any manner of wheat or barley mash, made up into a kind of porridge, alica or puls, into which perhaps some raisins, scraps of meat, or a few vegetables might be added. That the Roman plebs had little time to sit around eating is quite clear--all our modern research has abandoned the notion of the "idle plebs" in favor of an urban population suffering heavily from malnutrition and disease, dominated by semi-itinerant day-workers who were forced to hunt for work or starve, and needed to be working pretty much all the time to be able to feed themselves.

Can you link to the sources where you have found that they are three meals a day?

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