I want to attach a priming tank to an existing set of pumps. the pumps take suction from an underground tank with suction lines having foot valve, thus normally pumps remain primed. I understand that generally a tank having 3 times the volume of suction line is used. But once I start the pump it will suck all the water from the P tank, IS THERE SOME WAY TO ENSURE THAT PUMP TAKS WATER FROM PRIMING TANK ONLY IF THE SUCTION LINE NEEDS TO BE PRIMED??

I have worked with priming tanks based on your "3x volume" rule and they work well.  The line from the sump should enter the top of the priming pot. Use a 180 deg bend on top of the tank and go straight down into the sump. In this way, if your foot valve leaks and air is drawn into the system only the pipe from the tank down to the surface of the sump can drain out.  The 3x volume remaining in the tank will enable the pump to re-start and draw the water up the suction line.  Entering from the top ensures that there is good turbulence in the tank and with time any air in the tank will be entrained in with the liquid and pumped out - leaving the tank full of liquid for the next time it starts.

About having check valves so that water is always available in the lines and forgo the priming.

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