Going back to the original question.
I would be concerned about having 3 snakes in 1 tub with a lock up. If 2 snakes were locked up and the 3rd decided to go cruising over them or entwined in them I would imagine it could do some damage to both the male and female locked up if they were pulled apart. Beside that thought, they may not hold locks due to the 3rd snake being in there and added stress.
I know some boa breeders will have a 1.2 breeding maybe some research into that may help.
I'm not sure if you would separate them when they were about to lie. Females get uncomfortable before lying and move around a lot. This is a stressful time for them which could be exasperated by the other female and causing stress to that other female. Then going to one female being on eggs while the other's getting ready to lie, I would imagine that female #2 would definitely be struck at.
Also consider tub size. Having 3500+ grams worth of snake in one undersized (I'm not saying that yours are, just generally speaking) tub could cause bent scales and unnecessary rubbing which could lead to infections.

Personally I would not embark in this particular new housekeeping.
But, we all want to be the best keepers we can. If experimenting can teach us a new, better way, great. Just as long as the animals are safe, healthy and as ‘happy’ as we can understand them to be.

Keep us posted and Good Luck!