How can I research the true identity of Druids by comparing the perspectives of Julius Caesar, Pliny and Claudius?
The Romans were quite chauvinistic, so it's hard to say how accurate any of these Roman accounts of the Druids actually were. But we can at least compare them and note where they are consistent or inconsistent.
I've linked some of the actual texts we have of Julius Caesar and Pliny (in English translation of course), so you can read through them yourself. I wasn't able to find any actual writings by Claudius about the Druids, but I have some historical background for you about how they were persecuted under the reign of Claudius and other emperors of that period.
Some of the details Caesar gives us are quite likely to be accurate: There's no reason for him to lie about them having a particular focus on astronomy and a single arch-druid of supreme power. (He could still be mistaken, but he's probably not lying.) Likewise, his distaste with their egalitarian attitudes toward women probably reflects that the Druids were at least less misogynistic than the Romans (though this is not saying much, seeing as wealthy Romans literally named their daughters things like Cornelia I, Cornelia II, Cornelia III), and could in fact have had progressive gender roles as we might think of them today.
It's when he gets to these parts about savage human sacrifices that he is probably being dishonest; we have no evidence the Druids actually engaged in human sacrifice, and it's more likely a libel Caesar used to drum up hatred against them.
Pliny actually had no direct experiences with Druids so far as we know, so he was really just pulling from earlier sources---including, very likely, Caesar himself. He spends a lot of time fretting about the human sacrifice, but like I said there's actually no evidence that this ever really happened.
Finally, Claudius was one of the emperors who focused most heavily on suppressing the Druidic religion. We have no evidence of him actually ordering mass executions or anything like that, but he did make their religion illegal and order the destruction of a number of Druidic temples and holy sites. Had this not happened and these sites were preserved until today, we'd probably know a lot more about what the Druids really were like.