My guess is the beer is overcarbed. I don't burst carb very often, but the times I have it took between 36 and 48 hrs at 30 psi and 40F to carb 5 gal to 2.4 vol. Lower temps would mean faster carbing, and 3 gal probably carbs faster than 5. I would suggest taking it off the gas completely and venting every time you think about it for a day or two, and then taking a sample to see where you're at. It is very easy to overcarb using burst carb methods, and fixing an overcarbed beer is a PITA.