You can use the 3% solution; however the maximum I dose is 1.5 ml per gallon once per 24 hours. When I have an outbreak I use an oral dosing syringe from the drug store and "shoot" that most heavily infested area. For the H2O2 to have the maximum oxidation (killing) effect the algae needs to full photosynthesis so I do it after the light has been on at least one hour or more.
Roy 75 Gallon, 2X55W AH Supply CF 8800K, 1XMarineland Doublebright, 2X Marineland 350 Magnum; 45 Gallon Tall, 96Watt AH Supply CF 6700K; 30 Gallon Long; Fluval F&P 2.0 36"/46W; 20 Gallon, 1X26W AH Supply LED kit; all with Press. CO2; (Calcined) Montmorillonite Clay Greater Seattle Aquarium Society (GSAS)