If i had two pfizer shots in march 2021, then delta in august, do i need booster? i have seen recent studies show infection acts like booster. eu regulators just warned too many boosters can weaken immune system. would this be like a 4th booster?

You have misunderstood EU regulators' COVID vaccine advice or seen unreliable reports. A third dose of vaccine does not weaken the immune system in any way. Philosophically, there is a case for holding off on boosters so that more vaccine is available in resource-poor countries. But if vaccine is available, 3 doses (i.e including booster) is now considered the norm for all people in the US.