.75" ID pipe has about .44 square inch effective cross sectional area. .44 x 3 gives you 1.32sq.in. effective total cross sectional area (CSA) which can be handled by a ~1.3" ID. This would be close and after the friction loss by the smaller pipe might be close enough.
Issue is in schedule 40 PVC the average I.D. for nominal 3/4 is actually .824 (.53CSA), the average for 1 1/4 is 1.38 (1.49CSA). You end up about .1sq.in. shy, or about 15% under sized. With vented drains like you've done I would have gone 1.5x safety for noise reduction, or 3x if it was in someone else's home. And aren't there 4 x 3/4" lines on that run?
3-4" pipe is pretty expensive for messing around with though...
Now there are 2 easy solutions that are off top of my head in the nano plumbing world that may work for you in this situation too.
1. You're going to pre filter those with sponge anyhow right?, once it's gunked it should provide adequate resistance.
2. On your 3/4 pipes, gently/slowly heat an area the size of a quarter with a mini torch, once it starts to yellow gently press a marble against it. This will give you a dent to limit flow where a gate valve won't fit. You can usually get away with heating a PVC once or twice, and cpvc up to 5 times to get it right without cracking it. This trick doesn't work on cellular core pipe but it's great for PVC and cpvc in the smaller sizes. It's how I tune venturis in my nano spray bars and match pump head across tanks.
Plumbing and sump is looking good, you've done allot of drilling and filling in a week. I'm still a bit jealous of your flood safe fish room. It would make building so much more fun and less math intensive.
.
.
