I'm sorry you're feeling like that, NoonooHead1981. Please look after yourself and make sure you get in touch with your doctor, or even a and e, or the samaritans , if you feel ever feel worse than normally at risk.
It might be good to contact the doctor, anyway, and tell them how you're feeling- doctors are supposed to be seeing people when necessary, now, it might be worth trying anti depressenants, for a bit, even. With your youngest so little, it could be worth looking at if you could have post natal depression, maybe.
You're having to deal with a lot of things at the moment, as well as the situation with everything being different , and a bit difficult with coronavirus, has got a lot of people down, also it is a depressing time of year, anyway, which might not be helping at the moment . So, really -it's not surprising you're not feeling great, at all.
keep reminding your self, of how well you are doing, looking after three children, and they will get easier to look after,as they get older, as well, so keep remembering, you are moving towards things being easier, on the childcare front , as well, all the time- because it does get easier, as they get older.
Try to look after yourself as much as you can, watch nice things on tv , or listen to music,or go for a walk- don't worry to much about house work etc, and try to make sure you're eating and sleeping enough.
It might be good, when you can, to try to go back to the drs, or dentist etc, to get some reassurance or advice, with your other medical conditions, but be nice to yourself, if it takes a while to get it done- you can only do so much at a time-and looking after three children is a lot!
I hope you feel better soon anyway, please look after yourself, and if at all feel at any immediate risk, please contact someone who can help-like your doctor-or even a relative or someone who might be able to help- I hope things get better for you , Noonoohead, look after yourself 
x