3 year old Suburban 6 gal DSI Hot Water Heater problems

So you are getting lukewarm water by heating with the electric element? If that is the case, you don't have the valves in the correct position, also check that the inside and outside showers valves are in the closed position.

Do not open the safety valve to bleed out air, use a faucet for that. Do check if the valve body is hot.

The gas burner is a separate issue, do you have a steady flame at the stove?