I totally understand op and my advice is grin and bear it until they leave! I am only half-joking too!
The thing is we are at the point in our lives when our dcs’ wishes are almost directly opposite to our own, even if our basic needs are the same. It’s bound to lead to various degrees of conflict and is a perfectly natural stage of life. You have NOT failed as a parent.
I think the best way to get through it is to accept it’s going to be hellish for a bit (interspersed with lovely glimmers of their past and future selves) pick your battles, be available for emergencies and be receptive to important bits of communication (which usually takes place just as you are going to sleep) but for the rest leave them to get on with it as much as possible, cook up some nutritious food every so often, let them do their own washing and cleaning and don’t take any of the rudeness personally and focus on some lovely hobbies if you have any time to yourself at all. This too shall pass!
Personally, I wouldn’t have a family meeting. I would tackle them individually. The eldest needs telling that you are expecting them to set an example for the others and you are relying on them to do this.
And let them sort out their own squabbles. Tell them once to put a sock in it and walk out of the front door if necessary and leave them to it! It’s nice if you have a dog that will come with you and listen to your frustrations!
Be a bit vague occasionally about when you will be back home and let them fend for themselves a bit. Why not go away for the night (spontaneously so they can’t plan a party!). Don’t be too predictable and bring your friends home too. In other words, it’s time to let your dc know that you have a fulfilling life outside of just being a parent to them.
Also, set a boundary about rudeness, lack of respect and laziness. Ignore any pettiness but don’t stand for major back chat or insults. Personally I think if you want your dc to have respect for you then you need to respect yourself. So be a good parent but don’t subjugate yourself to them too much or become their maid! Talk to a friend or or counsellor if you need a bit of support.
You usually see vast improvements when your dc leave home and realise that fending for yourself isn’t as easy as they think! Good luck op! I am in awe of you coping with three so close together!