Help! 5mo sleep still eats every 3 hours.

My (nearly) 5mo wakes up at the same times every 3 hours at night. He’s 18lbs, doesn’t have reflux, and isn’t fussy. I’ve tried using a paci instead of boob, shortening feedings (I take him off by 30min,) letting him get to crying stage before feeding, and sleeping in his crib in his own room (same place as naps). Nothing made a difference. He wakes up at those same times in 3 hour intervals like clockwork. Babywise didn’t help, and we’ve had a great night routine for 3 months. I haven’t slept in more than a 3 hour stretch since he was born, and I’m so exhausted all the time I could scream. Had anyone had any luck de-regulating their expectant baby? How can I elongate the stretches?