Building a system which involves an increase in internal pressure via an air tank in order to extend a series of telescoping tubes, and a subsequent full release of pressure back to atmosphere. For safety reasons I need the release to happen due to the internal pressure reaching a certain point, and for the exaust valve to stay open until pressure has been fully equalized.
The system is carry-able and small, and operates at low pressures (<50 psi most likely).
If the valve being used is a solenoid valve, "3 way" is a bit of a misnomer.
What it sounds like you need is what is commonly known as a a 3/2 way solenoid valve.  This valve will have a P port (to which your source of pressurized air will be connected), an A port (which you connect to your device, and a vent.  When energized, P will be connected to A and you'll pressurize your device.  When de-energized, P will connect to vent.  This is the standard  gate valve used for single-acting actuators, which your device would seem to be.
3/2 way solenoid valves are cheap and plentiful, and are available in a wide range of sizes (Cv values, i.e. affecting how fast your actuator will pressurize and de-pressurize).  They are available both pilot operated and direct acting- a pilot operated valve for your situation will be cheaper and will take less power.

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