Both the Droids cartoon and the DHC Droids comic series have really always been in conflict with the rest of the Star Wars universe, whether canon or EU. I'd have to find it at home, but the first SW Encyclopedia had stated C3PO had been over 100 years old, which would allow the cartoon or comic to fit into his life. But the adventures conflict with what we know from the movies. Though I just got an interesting idea, bear with me...

This is all on the basis that there is no Empire in the Droids comic series (easily squelched I'm sure, but I dont have the comics here at work)...

C-3PO and R2-D2 met BEFORE The Phantom Menace as that is where their adventures in the Droids comic series take place. Anakin if you recall builds C-3PO from old parts, perhaps these parts are from his former life, somehow stranded on Tatooine. As for R2-D2, we don't know anything of his life prior to TPM other than he apparently served in the repair crew for the Queen's ship. What if R2 and 3PO were together long before this and got seperated after their comic adventures and get reunited in TPM, with 3PO not remembering and perhaps R2 not sure how to tell him or maybe not even able to tell him.

Now as for the Droids cartoon, that could still take place between Episode 3 and Episode 4 as the Empire exists. There's nothing to say for the moment they spent the entire period in the ownership of Capt. Antilles. Perhaps he lost them, which explains why they are looking for a new master at the beginning of the Droids cartoon series. They eventually make their way back to Antilles with plenty of time to spare. The only kink in that armor is that in the cartoon we see an A-Wing Fighter, which I believe EU states wasn't created until after ANH. But if you ignore that for a moment, I think this timeline can fit together nicely...

* Before TPM - DHC Droids Comic Series. R2/3PO get seperated, R2 on Naboo, 3PO in parts on Tatooine.

* TPM - they meet again

* ROTS - given to Antilles

* post ROTS - get seperated from Antilles and the cartoon series begins

* post-cartoon/pre-ANH - return into the service of Antilles

* ANH thru NJO - alongside Luke Skywalker