My DH works from 2-11pm, is usually in bed about 1am and then sleeps until 9-10am. My DD is quite a bit younger than yours, but if she's up when I shower I just take her into the bathroom with me and she plays with her toys (or empties out the draws). Not sure if that would work for a 3yo as an alternative to showering with you.

My mum worked shifts when I was a kid. Dad played cricket back then too, so he'd take us with him if it was the weekend. My grandmother would watch us while he worked during the week. I just remember having to be really, really quiet. Not so much an issue for me as I've always been fairly quiet anyway. But my brother would often be sent outside (we had a big backyard) to burn off some energy. It was rare that we actually bothered Mum though. Dad and Mumma always stressed more than she did.

If your DH is finding it hard, a granny flat seems like a great idea. You can always find another use for it once the kids are older and at school.