I've been reading their accounts of Norse pagan practices and I've noticed they attest to remarkably similar things sometimes. Their accounts of Uppsala seem quite similar for example.
And basically wondering are they all completely seperate sources? Or did they read and influence each others accounts/get their information from the same sources?
I guess essentially what I'm trying to establish is how reliable their accounts are.
If they are all completely unrelated then we can probably take a lot of what their saying to be true. But if they read the same sources then it's possible it's just the perpetuation of old myths.
In short answer, at least Saxo almost certainly knew and made use of Gesta of Adam (Mortensen 2018: 49-52).
Of three variants (ABC) of medieval manuscripts of Adam of Bremen's Gesta, a manuscript belonged to B (Codex Soreanus) had came to Denmark during the Middle Ages (probably by the end of the 12th century, though not lost by the City Fire of Copenhagen in 1728). Sorø Abbey in Sjælland had this manuscript, and the abbey had been founded by the Hvide family and reformed into the Cistercian monastery no other than by Bishop Absalon from the same Hvide family, patron of Saxo Grammaticus (Cf. Kristensen 1975).
Saxo's another indigenous Latin source, Roskilde Chronicle (Chronicon Roskildense) (customarily dated to 1138) had also already made very heavy use of Gesta of Adam, in order to establish the Christian past of Denmark against the possible interference from Germany (!) (Gelting 2004; Id., 2011).
It is also worth noting that we didn't probably have an independent archive attached either to the cathedral or to the monastery (added:) in Sweden at least by the middle or late 12th century, in contrast to Denmark and Norway.
As for the case of Snorri, it is unfortunately a bit difficult to offer an definitive answer.
While Snorri certainly rarely cites Latin sources by name, Adam's work had probably been known in Iceland long before Snorri: Mundal's article suggests that Ari Þorgilsson's Book of Icelanders (Íslendingabók), the oldest historical writing from medieval Iceland, indeed borrows its basic framework of the narrative (chapter by individual bishops in its later part) from Adam's Gesta, successive lives of the archbishops of Hamburg-Bremen (Mundal 1994). In other words, Adam of Bremen is, so to speak, "Adam", ultimate originator of the whole genre of historical writings (including sagas) in medieval Iceland. Thus, it is very difficult to confirm the existence of traditions definitely independent from Adam on the relevant topic.
On the other hand, Snorri did not probably refer to Saxo's work.
As for the circumstances in Sweden and its alleged paganism, scholars also often come into question on whether the Icelandic authors really know much about the topic or just express their general prejudice against much less civilized Swedes (this kind of prejudice was also shared by other Scandinavians like the Danes as well). Lönnroth concludes that "......These narratives [skaldic poetry and sagas] are interesting, however, as evidence of the condescending attitudes that skalds and sagawriters in West Scandinavia towards their Swedish neighbours (Lönnroth 1996: 157)."
Argument of the following previous posts of mine is mainly based on the early 12th century Danish hagiography and dating of runic stones in Uppland that suggests the presence of Christian local elites at least since the middle of the 11th century, against Adam's notorious account:
References:
Adding onto this I’m curious as well whether Snórri was aware of the Graeco-Roman classics. I’m trying to write a summary of the common Germanic mythology and I don’t know how much he takes from other sources (and thus how accurate his portrayal of Old Norse paganism is, and how divergent it is from the common Germanic base).
Are there texts and books that describe Proto-Germanic mythology accurately, as historically as possible, in detail?