3 on....1 off repeat. Calves/abs every non leg day.

1-Chest/back all angles 6-8 sets 10-12
2-quads/hams..6-8 sets 15 full range
3-Bis/tris/delts

Off
Repeat

I did a 5 x 5 protocol a few months back and loved it, I got so strong. But I didn't really put on much size. with the 5X3X1 I would think even less actual size would be generated.

I went back to a bodybuilding routine and immediately grew. I don't do the one body part per day thing tho. I focus on my weakest points and work them twice a week at least. I keep my rest periods short, 60 seconds and hit it hard.

Here is typical back routine for instance -

Pull ups 4 sets of as many as possible - with a spot at the end to get the last few.
Deadlift. 1 warm up, 3 sets. I still like to go heavy on these as I feel my whole body benefits from some heavy lifting.
Rows. I'll do two types of row. T bar 3 sets, and seated cable rows 3 sets. I'll make small adjustments with positioning and grip to hit my whole lats.
Then I'll do some good mornings or hypers to hit my lower back. 3 or 4 sets as many as possible.

No two workouts are ever alike.

I don't really count reps because I think the mind shuts off when you say you are going for 12 and could actually easily get 15. So I go for a weight that I can get at least 10 and go from there. Sometimes its 25 reps if i do a rest pause.

You will certainly grow if you make the switch. Just don't be bummed when you can't deadlift 650 lbs in a few months.

When you train heavy weight low reps in the 1-3 range your mostly training your neurological system, your brain in a way gets more work then your muscles because of the fact that they aren't under tension for enough time, when you train in the 4-8 ( little vague here rep ranges and effects will overlap) you train for myofibilar growth, which is the actual growth of muscle fibers, therefor you can be strong and big and solid. NOW when you train in the 8-15 range you are training for sarcoplasmic growth, which is basically taking your muscles and filling them with water, glycogen, creatine and other stuff, so the muscle will have that nice rounded look with out actually bein any stronger. It is impossible to train for just one type of growth with out the other one gainin just a little but you certainly can aim for one of the other. Now the good news is I'm assuming that you have a great base if you've been training powerlifting style for so long, which is nice because you have already put that work in. You should now train in higher rep ranges(8-15) with focus on TUT (time under tension) you need to be really careful that the muscle is under tension for long enough to fully work it, it's basically they opposite of what you have been doing, you don't wanna do the reps as fast as possible like powerlifting, try a 1 second concentric ( lifting ) and 3 seconds negative (lowering). Make sure you are feeling and squeezing your muscle rather then just going thru the motions. As far as splits go that's up to you an how you want to work it, that's not as important, just make sure your getting to the gym whethe it be Monday or Thursday, you'll find what split works best.