The 3% isn't as pure as 16% food grade (which I can get at my health food store) or 35% food grade.
I can get the 3% at costco, 2 large (32 oz or so) bottles for under $2 a bottle. I use the entire bottle in one bath.
First time I did a H2O2 (Hydrogen-Peroxid) bath I was quite shocked with the results (totally unexpected). I was just about to start my cycle (that day, in fact) and for the 1st time in YEARS I had no cramping (it had been severe), nausea, or the ups and downs in flow. Amazing!! When I told my naturopath about it, his response was even more amazing (I expected a "hmmmm", but instead he asked if I used food grade h2o2).