The basic mechanics of the games are the same. Once you understand:
Shields
Food
Gold
Happyness
Defending your city
Improving territory
You can have very enjoyable games with either of the 3.
Civ3 adds culture which is an aspect you can either play hard or somewhat ignore, after all if you're focusing on Happyness you will build many of the things that boost your culture (basically if you really put importance on culture, you'd want to build them sooner... but unless you play on unrealistically hard levels you probably won't suffer drawbacks of not playing a culture game, you just won't see the added benefit).
Combat is similar but different in the 3 games. Civ1 was alright, imho. Civ2 got much better, in a bad kind of way... they made it more realistic but that took some of the fun away.
Civ3's combat is less realistic in terms of result of individual unit attacks, but does a much better job overall of modelling the true hardship involved in waging war.
When folks tell you Civ3 is harder, I think they'd all agree with me that what they're saying is that the COMBAT is harder. The AI is more proficient, less predictable and definitelly more challanging.
If those 4 aren't all pluses, I dunno what is.
Also if you skip the previous versions of the game you won't fall into the trap that many Civ2 vets here have... like expect ridiculous Civ2 production from faraway and/or conquered cities, and will see the corruption and waste for what it is... an additional layer to the game.
Definitelly do the scenario.
Definitelly read the manual.
If you read no other chapters read the first one and the two about "managing your city" and "managing your empire"
You should also read the one about units.
Ah.... to heck with it... you REALLY should read the whole thing
My vote is 100% for Civ3.
