Hi S.,
I too am a married "single" Mom. My husband works for an airline and is gone at least 5 days a week, sometimes up to two weeks at a time. It is certainly not easy being the parent responsible for everything and then adding a job on top of that. I found that my husband had a tendency to be so removed that he would come home and not even know which clothes fit which child...meaning that when he came home I had to guide him through simple tasks. But the worst part was that anytime I wanted to take an hour or two to go shopping or see a friend (without kids), he whined about it. It took a little while for me to understand it, but I realized that he honestly didn't know what to do with the kids while I was gone...and he felt threatened by the idea. To me, this was simply unacceptable and unfair, so I slowly started taking time for myself away from the kids and him - this was just an hour or two here and there. He soon learned the ropes with the kids again and now does not complain if I say, "Honey, I need to go do this, I'll be gone for a few hours, do you mind?" I even got to go on a 4 day vacation with a friend to Cancun! He did complain about that, but it was jealousy more than anything else.
It is not easy - I know - but he has to understand that when he is away making good money (he is also doing something he loves, has time to work-out, gets his mind clear before coming home) you are home doing the same thing, but with 3 kids in tow...and have absolutely no time for yourself to take care of your needs - working out, friends, and just a little low time.
You can also try looking for a local Mom's co-op which may offer childcare, fitness, and babysitting.
Best of luck!