[SiEgE]

I think this could be a good idea NOT to prevent player from going the 3rd spy, by making some kind of efficiency test. Imagine the next situation:
1. There is already 2 spies in team.
2. First spy is doing nothing but standing near enemy dispenser with his CnD out.
3. Second spy is trying to take down the enemy snipers.. he fails often, but sometimes he gets a few stabs by killing some noob snipers.
4. Some guy joined the game, he is really good as spy. Instead of forcing him to play as any other class, he can prove his effectiveness with his points.
Efficiency test starts here. For 2 minutes, game start counting all the points those 3 collect. Once timer is out, server forces the worst spy to change his class.

Lets take a close look at the result:
New guy - 6 points
Sniper-chasing dude - 2 points
CnD dumbtake - 0 points

CnD spy will get his class changed randomly. Also, he can't start efficiency test as spy for like 5 minutes(unless 1 of the 2 spies leave the team\change class).

Also, you can't start efficiency test if there's already 3 spies.

Same thing goes for all the classes, but with it's own limit.