I dont see why 3 races wouldn't keep the game going for long. To use an example lets take horde mode in gears of war 3 and compare it to mass effect 3's multiplayer features.
- Assuming ME has 3 factions to fight against, and each faction has the same number of different enemies, the number would come out at 21. GOW has 21 enemies + 6 bosses. Many of those 21 enemies are duplicates however, and the only difference between them is a different weapon layout, and a few of them can hardly be considered enemies (wild tickers, polyps, and headsnakes who are actually a part of another enemy)
I would like, and hope, for more than 3 factions to fight against as much as the next guy, but saying that ME's multiplayer wont be going on for long because of lack of different enemies to fight against is just not true.
You have several different classes to pick between with different skills to shift up gameplay, nearly as many enemies as GOW, will have more than the 2 demo maps obviously, have equipment in the form of various ammunition and armor boosts. GOW horde mode is still running and thats 6 or 7 months of its release.
I do realise that GOW also has several other multiplayer modes, but the popularity of Horde is still unquestionable.
Also, I would really hope they dont mirror enemies as some people have suggested here, otherwise I dont even see the point of fighting vs different factions except for cosmetics.
My guess is that Reapers, Geth and Cerberus are going to be the 3 factions, with possible DLC upcoming. Collectors would not have much of a point since they would be more or less the same as reapers. A mercenary faction could be possible, as it would allow for a diversity in races to fight against.