What do I need to do with a prednant gerbil ?

i have three gerbils two smaller onse and a fairly large one all from the same littler which I originaly thought were 3 females, but last night the larger gerbil was trying to mount the other two. what i saw was .. the female gerbil tapped her feet very quicky for about a second then jumped on to the larger gerbil then hopped off again. then the larger gerbil mounted the female and sort of groomed the females back while on top. the larger one also tried this with the other gerbil but she wasnt happy and ran of .. is this deffinatley mating and if so what do i do !!!!