So i have this code:

when I receive [Start v]
repeat (3)
replace item (random v) of [list v] with [2]
end

And whenever i press on the script it works, but when i broadcast it using a sender nearly the whole list turns into 2 instead of just 3, ive took the broadcast out of the scipt it was in and clicked on it alone and it still did the same thing

It would be easier to help if you shared and linked the project.. But my guess is that you have clones of the sprite that is receiving the message. Clones respond to broadcasts too.