I think you are in a difficult stage where the youngest is still quite young to be left that much, and the elder two may be up in their rooms on screens. Getting outside is what you are doing with them already, but it's tiring for you.
I think just keeping going, and having little tiny breaks for yourself is the way to start. Leave them for 30 min, let them know you are going out and then just go. It could be to the shop, it could be to sit in a cafe, go for a drive, just get the idea in their heads it's ok for mum to leave the house, she's a person too and if you feel anxious about it, that's ok, but it will be happening anyway.
You can then work this up over time.
Also, can then start doing some more independent jobs- again running down the shop for you, carrying in the shopping from the car, doing small tasks.
I think as a parent who has been left by the other parent so completely (mine was through bereavement), it's easy to end up overcompensating all the time, trying to make their lives lovely and less stressful and to make up for the fact dad has disappeared, but I don't think you can probably do that all alone and it can end up making you over-do it on the parenting front.
With three teens, you are stretched, and so building in small steps to independence will pay off in the long run. I thought one of mine would never ever leave home, they are now living away from home and are not even coming back for the holidays as off doing interesting things. We are still close, talk a lot. It will happen for you OP as you are laying a great foundation by letting them be close and secure, but over time it will be ok for them and you to be slightly more independent.