A class limit of three doesn't sound all too unreasonable for those classes (I'm not sure about restricting the Heavy, but more on that later).
Although I must say, three spies isn't much better than four (or five, but I've never even seen it come to that on our servers). I'd still urge at least one of the spies to reconsider his choice of class.
I don't get the amount of complaints about the Heavy (with a Medic) being such an unstoppable force. If your team can't handle a Heavy/Medic combo, you're handling it wrong.
People need to remind themselves that they're not the only person in a team. If you're alone and see a Heavy/Medic pair, run back to get some back up from teammates.
I've been frustrated with Heavies as well, but as far as I remember they were equally as hard to deal with throughout the history of TF2. Why is it that only now people start complaining?
A team with 4/5 Heavies doesn't perform as well as a team that had their five Heavies switch to, say, 2 Heavies, 2 Soldiers and a Demo.
Anyway, most of my post is about the Heavy balancing, which shouldn't be discussed in large here, but I'm just saying that it's not a class that needs limiting.
Also, NmD, there aren't any specific choices in the poll, so feel free to say why you'd like certain classes restricted in a response. That's what this topic is for
.