I had access to some 30% industrial grade H2O2. I cut it to approximately 5%. I really think the 3% will do just fine like it is. My brother went to a beauty shop supply store and got some 40% I believe.
I do not think the concentration is all that important. Just not too strong. We are having a drought here and I do have to water regularly, so I follow up with the H2O2 (Hydrogen-Peroxid) since it evaporates soon anyway.
One other note is plant disease. I have 2 early girl tomato plants that were 2' high when I bought them. They are the only tomato plants with blight. All the other tomato plants were sprayed from their small beginning. And blight is a real problem here.