Right ok, the bath situation..... 
So I'm guessing that the aim long term would be to get a bath-feed-bed thang going on, even if the bed bit is a little ambitious yet?
If you were to put both twins in the bath with dd would that not require you and DH to be there to hold one DT each? I know what you mean about the p shaped bath - ours is the same. Maybe I'm missing a trick though as to how one of you could do all 3 together? I'm intending to bath ds2 separately until he can sit unaided and then he can go in with ds1.
So one option would be to bath dt1 then feed dt1 then bath dt2 then feed dt2. I guess though there is a fair chance that dt1 isn't going to be nice and quiet while you feed and bath dt2?
How often do you bath dd? I would be tempted, if DH is not home to only take on bathing DD or the twins. Or indeed if it was a bad day, none of them!
Even if DH is home it might make sense to only do the twins or dd? Or does dd have a bath every day? Could you move her bath earlier or later for a few months until they can be combo-bathed?
If DH was home and you only had the twins to bath I'd get him to do the bathing and you feed each twin as they're out of the bath. That's what we did with ds1, and what we do with ds2 if DH is home now.
I'm not being much help I'm afraid. I just wonder if, even of DH is home, bathing 3 of them between 2 at the same time is too much. Because ultimately one person gets left to bath the twins, and that's always going to be tricky.
I can only bath ds1 and ds2 in the same session because ds1 is that much more self sufficient. So he either plays, gets undressed or 'helps' while I bath ds2. Then I put ds1 in to play while I dress ds2 (obv still supervising ds1). Then I give ds2 some tummy time and do the washing bits of DS1. Then get ds1 out and dry and dressed and do story / bed etc. but that's the tricky bit if on my own as there is every risk ds2 might fall asleep while I'm getting ds1 in bed.
But as I say that only works because ds1 is more self sufficient. I doubt I could even easily do that if I had ds1 at 19 mths like your dd. And I only have one baby to bath.
I'm considering starting the baths earlier if I'm on my own so that I could bath ds2, then ds1 then get ds1 out and into pjs but then leave him to play while I feed ds2 and then put ds1 into bed so I'm not rushing through his story.
Anyway, in summary I reckon that the trick has to be to bath as few children as possible at the same time. No need to make it more difficult for yourself than is absolutely necessary.