Hey Zorfox, thanks for the report!
Algae not initially killed will, for a time, adapt by producing more of its natural defenses to both H2O2 and Excel. So each successive treatment will be a bit less effective. Best case scenario, it will require more than 3 treatments at 1/3 dose, to get the same effect as a single treatment at full dose. Worse case, the algae will grow sufficiently resistant that you have to increase dosage or frequency to see substantial continued improvement.
That's why I prefer to use the largest H2O2 dose I can reliably get away with, and a sudden spike in Excel rather than continuous overdosing. Still, "chipping away at the stone" can work too. If you prefer to proceed this way, I encourage it, and would love to see future updates.