The holidays must be quite full on if you're at home.
Because it's much easier to be cheerful about other people's lives, I'm going to offer an alternative perspective 
Isn't it good the 18 year old is volunteering? They need to grow up and start making their own way not he world at that age, even if if it is hard to take as a parent. I doubt he's going off you. I don't think children go off their parents, they just annoy the hell out of each other sometimes! You probably were grouchy yesterday by the sounds of things, but that's OK, he's old enough to cope with that!
A walk or run sounds better than just being glued to her phone in her pyjamas all day, but I kind of think they really need the holiday and they won't have that chance often in life, maybe just let her get on with it? As long as it's not creating extra work for you.
The youngest - are you feeling like you need to chase him up? I'm pretty sure my parents had zero idea of the status of any of my coursework! But could you sit down together and figure out a plan? I'm not sure they actually get taught any planning skills.
What would make the holidays easier for you? More time out by yourself? Taking it in turns to make lunch and clear up? A few days out that everyone can agree on, or some one-to-one time with each child doing something you'd both like?
My mum has just phoned me demanding to know the exact details for something that's happening in three weeks time. It's really annoyed me, but I know I'm being totally unreasonable. I need to phone her back wren I'm less annoyed!