Alternating system of 3 pumps

Hello
I have been thinking in some way to operate 3 pumps in the same way (I have schneider modicon m340 PLC):
-the pumps feed a tank.
- As maximum it can work only 2 pumps of the 3 available.
-The operation of the pumps has to alternate depends on the operating time
I tried to compare the running time of the 3 pumps by timer but I could not. Can you have any idea for it?
Thank you

http://www.plctalk.net/qanda/showthread.php?t=98315

http://www.plctalk.net/qanda/showthread.php?t=22500

http://www.plctalk.net/qanda/showthread.php?t=32476

The above threads may help you out with the logic.
Regards,

Thank you
Any suggestions to apply the runing time of the 3 pumps .

It’s generally accepted in full triplex pumping systems that in the long run, pump wear and tear evens out without needing to get overly aggressive about trying to force it to happen. A simple way of dealing with it, mimicking the way it is done by humans with simple triplex panels, is to keep a running log of run time and start counts, then if one pump deviates more than about 10% higher than the average, take it out of rotation for a while.