Dont ask me wat the reward is...I have a problem theres a cap on scieence so i cant make .....and so on... but the question is that can i make all civilizations in my scenario have 3 towns at maximum

Make start units 1 and 2 both settlers and change scout unit to settler in general menu, make all civs expansionist in civs menu. Then make settler's prerequisite - NONE - in units menu. Should work in theory. You might also have to make scouts available to all civs but don't quote me on that.