Thank you, WinterisComing. Your assessment of my situation WRT to DS3 is correct.
I am trying my bloody best to go back to work, but the lack of SN childcare for a 13yo with Autism is a HUUUUUUGGE sticking point. DD is not able to stay at home alone, it is not safe. In fact, my 9yo DS1 would be safer and more able to stay at home than DD is - and none of you would expect me to leave a 9yo at home alone for up to ten hours a day, would you? So how is it any different being expected to leave a 13yo with SN at home alone for between 3 and 10 hours a day?
Because even if I finished work at 5.30pm, due to my DISABILITY I have no choice but to rely on public transport (which, on NMW means the bus, taxis are too expensive). I would have to catch a bus to the nursery, pick up DS3, catch another bus to the after-school club and pick up DS1 and DS2, and THEN catch another bus to get to my house. I was getting in at 7.30pm after picking DD up from the SN childminder - so I can assume that even going straight home from the after-school club, I wouldn't get in until 7pm, so she would be alone for up to 3 hours EVERY evening. When she is functioning at an 8yo level. 
(I am not allowed to LEGALLY hold a driving license, due to having uncontrolled epilepsy, I would have to be seizure free for over a year - and I haven't been seizure free for that long at all since my diagnosis nearly 8 years ago).
I am BLOODY TRYING MY BEST!!!! And yes, I DO have to pay my Ex-P back for the electric, because it is over and above the maintenance he pays me. The house move may make the situation better WRT to the bus fares, or it could make it even worse. I have a Prohibited Steps order against me barring me from moving DS1's school, or moving away from the estate that both me and his father, my Ex-H (not my Ex-P) live on. Which also happens to have the highest rents in my town - and I may be moved FURTHER away by the council. In which case, as I cannot move DS1's school, my bus fares would be MORE rather than less. It's SHIT. But I am BLOODY TRYING!