I have 3 pregnant guppies, how will I know when will give birth?

i got 3 pregnant guppies at the local pet shop. they are huge. their gravid spot are very very dark. their bellies are bulging out. how will i know when they will give birth. as of right now i have all 3 of them in a separate tank. and its moderately planted. should i put them in breeding tank or breeding traps?