I agree with Borek, in that is it most likely oxygen.
You can verify this, by measuring out a specific amount of NaCl and your 3% H2O2, heating the reaction and producing the same results as before (do not spill it though). And then boil away the water and see if the same amount of salt remains. Do not boil it to vigorously though. If the same amount of salt remains, you know it was a reaction of the H2O2 with either it's self or water, making O2.