There are some huge differences in terms of development between an infant session and a 3 Month session. 3 Month old babies do not sleep as much as infants and are not as squishy and flexible so there is pretty much no chance of coaxing them into positions. They DO start to have good neck strength so you can prop them up in a nursing pillow or with towels rolled under their chest and get some great shots of the cutie discovering a new point of view.
Many 3 Month olds are most comfortable on their backs so make sure to get shots from different angels and always make sure your camera strap is around you neck when kneeling over them.
Involve Mom and Dad as much as possible. They are the best at getting great smiles from their little ones and capturing the moments when they are just interacting with their babies make the most precious images. Guide these moments toward the best light available.
Always get a shot of the parents with their faces right next to babies. There is always so much love that shows through in these images!