Hugs OP, totally understand your need for work and to have a bit of a life of your own. It's not easy being a single mum of 3, one with SN, it sounds like you are doing great. Bear in mind that this situation will probably only be for a few years while the job could be for 10 years+... your middle boy will get more mature (he may surprise you, they can grow up a lot when starting secondary) and need much less care in the holidays plus your youngest will start school soon. Just a few ideas, sorry if some of these won't work for you (as you don;t give details of your oldest boy's needs):
-Any family, friends, parents of your boys' friends or neighbours you could beg some help off in exchange for favours/childcare in return, even if this means them coming to stay with you or you shipping the older two off to them? I have a teenage cousin with HFA and various family members stay with them over the holidays and he goes off to stay with Granny for a few weeks too, although he's not safe to stay at home alone it's no bother keeping an eye on him (he needs help with meals etc and someone to be there in an emergency but he mainly plays on his console most of the day). It's totally different IMO to looking after younger children who need actively entertaining, so people may surprise you with willingness to help, esp if you bribe them...!
-Would your eldest/middle one be OK being left for a few hours, then someone looks in at lunchtime and makes sure they have lunch etc, then left again for the afternoon? You would need to make sure they can cope in an emergency of course. If you could slightly shorten your working day and make up some hours in the evening this system could work as they'd only need to be alone a few hours at a time (if your work is amenable of course)?
-Would the youngest's sitter consider having one or both of the older boys in the holidays (don't know how this would affect her ratios but again it could be easy money for her if they are largely self-sufficient)?
-Do either of your older ones have hobbies or interests you can look at holiday courses for? They may feel they're a little old for holiday 'clubs' but be up for doing a football course or stage school or similar in the daytime? If they can persuade a friend to do it with them this can massively increase their enthusiasm in my experience! You could even look at 'stay away' camps like PGL, perhaps especially for your middle one who is a good age for things like that? Any specialist provision for autistic teens for the older one in your area?
-Remember you have annual leave and are entitled to take some unpaid parental leave in blocks of a week too, so don't feel like you need to find 6 weeks of childcare in one block, if you could find someone to cover 2 weeks, 2 weeks of courses/camps and 2 weeks of annual leave you are sorted!
Good luck!