first off you should find the cause of it. Be it poor tank conditions, high nutrient levels, low co2 or high light.
3 day blackout might not do much, i mean it will get your water lightly clearer but once the green water spores are in the tank, they will just multiply again when you give the tank light. Normally they suggest 5day blackout. But a blackout isnt always healthy for the plants. From personal experience, i had alot of plant die off after only 3day blackout. After that i gave up on it and tried other things.
I upped the filtration and tried Accu-clear along with water changes. After that the tank got clear again and i havent seen green water since then.