Scratch

ok, i have 1 sprite with 3 costumes. each of them are under 1 green flag, the code is, when green flag, always if key 1 is pressed and so on, then for the second costume, the same thing but key 2 pressed, so when i play it, and change costumes i can see the previous one flashing behind it.

Fullmoon's answer will probably work fine. Sometimes, your scripts will contradict each other, so it'll rapidly switch costumes back and forth, resulting in the flashing you see.