If you mean by three-valve manifold, then If you apply excessive pressure to your differential pressure transmitter, you could damage your instrument. This is known as over-r
…anging the transmitter. A three-way manifold valve is a device that prevents the instrument from being over-ranged. It also allows the isolation of the transmitter from the process loop (an option which could be used generaly for maintenance or re-calibration or fitting new equipment). It consists of two blocking valves and one equalizing valve. The blocking valves are for both high pressure and low pressure sides. During normal operation, the equalizing valve is closed and the blocking valves are open. When the transmitter is put into service, care should be taken to ensure that the high pressure is never applied to one side of the Differential Pressure Transmitter. To put a DPT into service using the Three-way manifold valve is to first close all valves. Then, open the equalizing valve to ensure the same pressure will be applied on both sides of the transmitter. Therefore you will have a zero differential pressure. Next, the high pressure block valve is opened slowly (to check for leakage from both sides of the transmitter). The equalizing valve is closed and this locks the pressure from both sides of the transmitter. Now open the low pressure block valve to apply the process pressure to the low pressure side and establish the working of the DPT. (MORE) 
