Three Pump alternating program for RSLOGIX 500

I work at the local water utility. We have a pump house with three pumps. The current slc500 RSLOGIX 500 program is set up for a duty pump and standby pump alternating system. This is how it works. Under normal conditions one pump (the duty pump)is running. If the pressure drops below a preset value for fifteen minutes the standby pumps comes on. Once the the pressure rises then the duty pump turns of and then the standby pump becomes the duty pump. The problem is that demand has increased so much that it is now necessary to have a second standby pump come on when two pumps are already running and the pressure is still low. I have an idea on how to do it but I am unsure how to write the programming for the alternating between duty pump to standby 1 pump to standby 2 pump. I figure if I could get a copy of a three pump alternating program it would help me a great deal. Help anyone.

see if the program at the end of this post gives you any ideas ... this system works on vacuum signals instead of pressure - but the layout of the program might be helpful ...