Not really knowing anything about the liquid and headspace volume, or the strength of your fermenter, I would recommend more than 3 psi. If your fermenter is rated for 10 psi that would be a safer bet to prevent a vacuum that could potentially damage your equipment.
If you are currently at normal atmospheric pressure and fermentation is complete, just pressurize your fermenter and cold crash. For your next brew you would attach the spunding valve during fermentation and set it to your desired pressure. pressure will build up as your yeast produce CO2, so when your ready to cold crash you already have pressure built up in there.