me and DCs father are on and off
Well forget the debt - if you're concerned about what's best for your children nail that one down right now.
Does he live with you? Does he contribute financially to the children or care for them in any kind of regular, useful way? It's time to take a cold pragmatic look at him, totally irrespective of your feelings for him which I'm sure are complex, and decide what good he is doing your family if any.
FWIW in my opinion, you have children, you are caring for his very young baby - he's either all in or he's out. I know you are young, and in an ideal world you would still be dating and breaking up and getting back together again and exploring your sexuality and your emotional boundaries etc - but you have young kids, and anyone you are on-and-off with they are on and off with too.
So you need to tell him that either he commits, you move in together and he helps you practically and emotionally to raise your children and better yourself for their benefit (e.g. caring for them an equal amount so that you can go for driving lessons etc, contributing to bills etc), or you two are over, for good, and he can start paying maintenance for and have regular contact with the children that are his. Sort that shit out before you do anything else. The last thing kids need is to be exposed to an unstable parental relationship and a will-o-the-wisp now you see him, now you don't father figure.
Beyond that... you have to stop talking yourself down. You come across here as someone who is self aware, extremely articulate and good with words. You spell fine. Your grammar is fine. If you talk anything like the way you write, you'd come across well at interview (apart from all the self-deprecation!!!) When you're ready to, there will be opportunities for you - careers advice at the Job Centre would be a good place to start.
And try to be realistic rather than visionary about the work you want at first - yes university would be a great thing further down the line (say when all your kids are in school/sleeping through the night and you have time to study!) but maybe just get in the habit of working first and establish your patterns, preferences and work ethic. I would hazard that you will need to be in some sort of regular work for a while to develop that 'get up in the morning and go in whatever else is going on' instinct which people raised in working families will just get but if you were raised in an environment where work and/or school attendance were seen as unusual or optional you may not have yet (not your fault!). Just start small, find a job you know you can do without too much stress (something routine with limited pressure) and cobble away at it, learn o rhythm and routine of working life, bring some money in to your family, then start thinking about what you might want longer term.
However, before all this... enjoy being with your tiny baby! At 3 months old you wouldn't usually be thinking of going back to work yet if you were on mat leave. Try and get your debts under control (speak to the free advice lines available) but don't worry too much about improving the situation yet if it's not in crisis - let yourself recover from birth and devote lots of attention to your little ones. use any free time you have to engage with the right services that will support you back into work when the time is right, get help reframing your CV etc.
So much time ahead of you. As someone else says, you'll still have a whole life to live when your kids are all grown up. Don't feel like you've mucked it all up, you've barely begun, and you've already got a head start on most people because you already have three beautiful kids. Hang in there OP! xx