I actually would make sure you keep the volunteering activity, it is probably a break for you, adult company, a chance to feel appreciated outside of being a mother and beneficial for your mental health.
Also is there room in your budget for you to get a cleaner once a week or fortnightly to lighten the load?
If you feel the relationship with your husband is worth keeping, as you are essentially single anyway but without any of the advantages, then I would have a serious discussion about his work choices.
He is behaving just like a single man with no responsibilities - you don't get married and have three children then rack off and work abroad on your own, indefinitely. Who does that?!
I don't know what career is so specialized that no version of it exists in the UK - even if he's an astronaut, the right thing to do would be to take a different job so that he can actually participate in the family he has created.
My husband, like many parents, remains in a job that is boring with no progression because it is secure and flexible so that he can put in a shift at home. Most people have to make some kind of choice whether to go all in 100% on a stellar career that may involve travel, crazy hours etc and those people often don't have children, and others would like to work but stay home to parent because that's how they feel they can best meets their children's needs. Your husband, having his cake and eating it whilst leaving you struggling, is selfish.
As others have said, I don't know why anyone would have a second or third child with a man who clearly wasn't pulling their weight and barely sees them, but you are where you are so all you can do is structure your life the best way you can from here.