They should just push back in. Make sure the plug part in the center is pulled (I believe) so that the plastic "fins" can flex inward when it meets resistance from the hole. Once it's in, push on the center to push those fins back out which will grab and hold it in.
On a side not, IMO it's easier to take all three underbody panels off when changing the oil. Leave them attached to each other and just take the bolts/pop rivets off around the side.