Hiya. I don't have 3 under 3, so can't help with that part. But sorry your DP isn't being very supportive
But for the evening meal- do you have a slow cooker? If you do, you could put on dinner in a quiet moment in the morning (or even have some of it ready to go the evening before and just add the meat in the morning). Or during summer you could have heaps of salads, so that it can be prepared ahead of time and just eaten when you come home.
Could you eat dinner with the kids a bit earlier and your DP eats after the kids have got to bed? We normally have dinner at 5.30 and I eat with them, if DH comes home early he eats with us otherwise he heats up the dinner when the kids are in bed.
Our routine with 2 of them is:
Dinner 5.15/5.30ish
Bath for both them at 6,
6.30- DH reads Ella a story and puts her to bed, while I BF Tom and put him to bed.
Good luck- it will be so worth it in the end, your 3 kids are going to the best of friends