A lot of people who use multiple relay servers ask whether it is best to set the servers in servies or parallel. I'll explain below:
Where, for example, you have one primary server and three relays and have relay one listen to primary, relay 2 listening to relay 1, relay 3 listenning to relay 2..
Primary > Relay 1 > Relay 2 > Relay 3
Where all of the relays are connected to the primary server:
> Relay 1
Primary > Relay 2
> Relay 3
It is best to set all of your servers to relay the Base server in parallel than having them in series. This way, if one of the relays goes off, it has no chance of affecting the others, but if you had them in series then if the first or second relay went down it would take all of the others down the line. If the Base server went down then they would all go down whichever way you did it, so make sure that you use a good server for the Base, the premium network would be ideal.
It is also best to make sure you reserve the relay IPs so that the relays can always connect.
Also I've noticed before that if one of the servers does fill up, listeners will automatically redirect to the next relay so you only really need to link to one of the servers - although it would be better to set up a playlist (pls or m3u) file and link listeners to that so that listeners connect more randomly and fill the servers up a bit more evenly.