If I'm the strongest in my server, I will use my weakest team, then my strong team, and if needed, then rez my strong team and use my middle team to do the 3 Xserver fights to get that reward
Then just wipe everyone on my own server using my strongest team. And that's assuming all the Xserver fights are losses, which usually aren't true for anyone but those who are from younger servers.
So no, 3 won't be enough if you actually want people to do Xserver for real, because even if worst come to worst, they can still just own their own server. With the EXPECTED loss count of only 1 to 2 in 3 fight... well....
If it's going to be optional, the requirement should be 5 fights, win or lose, and/or 2 wins in Xserver mode. Or frankly it won't carry any risk for your targeted audience. reward should always come with risk.