2 fast, stable, robust ones - can be choosen randomly from a list of fast, stable, robust ones
1 random from the not so fast, not so stable ones

or something similar 

 Hard to measure fast and stable. All relays get slow from time to time, and they all go offline for minutes/hours at a time.  

 Yes, a statistics will be needed but simple average of response times and uptime against the relays can be a pointer.