I get bubbles in the beer line when I overcarb. Also, I carb at 30 PSI for 2 days. 3 I find to be too much.
Finally, if you used a barbed coupler to connect your 3' to your 5', that can cause problems too. It will act as a venturi section, dropping the pressure of the beer a lot as it passes through the restriction, causing CO2 to crash out of solution. You want a single, uninterrupted line. I personally use 10', as that's what I need, but your mileage may vary.