Why 3% hydrogen peroxide? Food grade?
3% because it is readily and cheaply available. And then when you dilute it 1:50, that makes it cost almost nothing.
35% is cost-prohibitive except for manufacturing activity including food processing. That level of purity is not necessary as far as I have literature on.
And then you know that pure hydrogen peroxide is rocket fuel — which I doubt that many will encounter in their lives — which of course is supremely toxic also.