Sleep schedule for 3-month-old is all over the place

My son just turned 3 months and some days he'll sleep 4 hours straight, other days he's up every 90 minutes. We're doing the SNOO and it helps but I feel like we're not following any real pattern yet. Is this normal? Everyone keeps telling me to "just wait it out" but I'm exhausted and my wife is even more exhausted. Should we be trying to establish a bedtime routine already or is that too early?