Is Conquests the one they sell on steam thats called "complete"? What are the differences
They're really just marketing terms. 'Conquests' was/is the second expansion pack (which also included the first expansion, 'Play The World'), intended to be added on to an existing 'Vanilla' Civ3 installation. IIRC, the original release of the '
conquests.exe' executable was v.1.15 (could be wrong, though: I also came to the Civ3 party pretty late!).
'Civ3 Complete' is the base-game plus the 2 expansions (at the final patch-levels), plus some additional content, all sold as a single unit—but when you play Complete, you'll (usually) be running the
conquests.exe executable (v.1.22). And if you have a downloaded version of '
InComplete', this is actually now the
only option, since none of the current DL-versions (that I know of) include the Vanilla
Civilization3.exe (v.1.29) or the PtW
Civilization3X.exe (v.1.27).
So you're already fully patched. And if you can play SP without crashes, and MP for 1:30-3:45 hours before it crashes, it sounds like your game is working fine — but the internet connection/Steam hosting service may not be. Could just be that traffic peaks at certain times, and that lag causes your game to crash.
But could also be just outdated code: Remember, the MP-mode for this game was coded back in 2002 or so (MP became possible with the release of PtW), and it was also originally designed to be hosted on another, now long-since defunct service (GameSpy), not Steam. (I vaguely remember reading that) The Steam version of MP-Civ3 appears to have been somewhat limited, e.g. it used to be possible to run MP-Civ3 over a LAN connection, so you didn't even need a remote hosting-site, but the Steam-version of MP doesn't allow this anymore.
Also, do you already know about the 'short-game' modes and victory conditions (e.g. accelerated production, regicide, elimination)? If not, setting those might help to end your games sooner, before you crash out...?
(NB As to where this question goes, it could just as well have been posted in the 'Civ3: Technical Support' subforum, but TBH it's probably going to be more visible here)