Wanted to add a note of hope - I only have two, they're 5 and 2, but it does get MUCH better when one goes to school, and when the younger one is over two. I have been feeling trapped, resentful, all of the things you describe - one of the reasons we're not having any more is that I can't face spending another two years marking time, not progressing in my life in any way.
That doesn't mean I'm a hard career woman, but just that children under two need so much attention that it's hard to feel like there's any of you left. Also summer holidays are hard, everyone goes away, all the activities stop, and it feels like it's never going to end.
But suddenly lately I feel like I'm coming out of a long fog - I'm applying for a new job, and feel like I might be able to actually concentrate on a job again! Admittedly it's taken a backslide over the last two weeks as DS2 is sleeping badly, but life feels possible again.
You need a plan. Get the nearly three year old signed up to a preschool, so that when the older one goes to school you also have some preschool time sorted for the three year old (if this is feasible from a financial point of view!). Take any opportunity to share the burden - playgroups, activities etc - where you get to meet other grownups and talk. I've made a fab friend at DS1s school who will have my two year old occasionally in exchange for me having her three year old occasionally, frees me up to do things by myself. Get one evening a week where you don't do bedtime, and do what you like - go out for a drink, sign up for an exercise class, even sit in a cafe by yourself and read a book!
Alternatively, fuck the plan, if that's making you feel too despondent. Just take this one thing from my stupid post, and that's that it DOES GET BETTER. I promise.