Arry
When you signed the original clause, I am sure the other 2 guys signed the same clause, and you all agreed to it at that time.
If 2 clients want to move with you, I expect its because they already have a working relationship with you. (and your 2 friends would be aware of this)
Surely, the best option is to sit down and discuss your plans with your friends.
Why are you assuming they will want to go to law, if its as straightforward as you describe? ( How would you feel if one of the others took 2 clients ?