I wouldn't consider Civ3 endangered - there's plenty of people who prefer it, as CFC shows [just look at the people browsing the C&C forum at any one time], and it still picks up a decent number of newcomers thanks to its low price. I know several people who prefer it to Civ4 myself, although not all of them are actively playing it at this time (schoolwork of various other games such as Empire Total War taking their Civ time, and one treacherously playing CivRev). I also know at least one person who prefers Civ4 to Civ3, and several others who play Civ4 and may or may not have played Civ3. And my roommate prefers Civ2 to any of them.
Which makes me realize that next year I'll be a Civ3 player living with a Civ2 player and a Civ4 player. Now I just have to get my final flatmate to become a Civ1 player and we'll have the whole series spanned!
Now if Civ5 comes out and is really good, then perhaps Civ3 could be in a bit of danger. Even so, my guess is Civ4 will go much quicker after Civ5 comes out than Civ3 will after Civ4 came out (it's still going strong now!). If it doesn't, I'll be pretty disappointed in Civ5.