I really need to shower, but am afraid to leave my 3 month old in the room with his sister without adult supervision.…

I just put my son on some padded blankets on the floor and kicks around and tires himself out

Bouncer or car seat in the bathroom with you, and leave door open so it doesn't get to hot

put him in crib?

take your 3 month old in the bathroom with you and strap him in his bouncer or something like that