Days and Nights mixed up?

I'm starting to wonder if the baby has his days and nights mixed up. After being so completely wiped out today. Not only did I have the baby waking up, but I had my 17 month old wake me up, and my 4 1/2 year old wake me up. Mommy really needs a nap today!
Little man wakes up every 3 hours at night (sometimes less). Usually he will eat and go right back to sleep. Sometimes like the other night he plays and laughs at you.
This morning he ate at 5am, woke up at 7 and went back to sleep by 8. It's 12:36 now and I stopped the swing and woke him up. So he can go hours without eating, that's not the deal. Why won't the child sleep through the night??
How do I fix this? He is 3 months old, most of my girls were sleeping through the night at this point. Only 2 were not. My 17 month old still wakes up once in awhile in the middle of the night. I love my babies but the first couple of months of their lives are the hardest. I'm one sleepy mommy today!