It's hard to give advice without knowing a few specifics. Like, where are you in the trilogy? If you haven't even turned in book 1, you are in a lot different position than if books 1&2 are out and you are writing book 3, kind of a thing.
The option means they get the first crack at whatever you write next. It does NOT mean you have to sell it to them -- just that they get a look. If they offer, you can always say no. You'll have to look at your contract to see the specific wording, but usually it is 30-90 day window, that can begin either whenever, or perhaps can't start till after delivery or publication of the last ms in the series. You really will want to read this carefully, as every publisher/option clause is different.
The Non-compete probably means you can't put out books with the same characters, or books so similar as to be detrimental to this series, within a certain amount of time. This should also be spelled out in your contract. But it shouldn't be a problem - after all, it is also hard to PROMOTE multiple books in the same genre coming out at the same time, you'd want to space them out just for your sanity I'd think! BUT AGAIN, read your contract carefully - maybe your non-compete is stricter than most.
There is no particular reason you can't fire your agent now, if you really dislike him/her so much. That would be up to you - do you want their input or help with the rest of this series, or not? From what you're saying here, it sounds like NOT. They'll continue to make $ off the royalties for this series that they sold and any subrights they sold, but unless you have an extremely onerous agency agreement, any future books should be out of their purview. And whether or not you take your unsold subrights with you when you go would be a convo you'd need to have with the agent.
Of course it might be tough to get a NEW agent if you don't have new material yet. But it sounds like this is a terrible relationship, which is more damaging than having no agent at all, IMO.
Good luck! And if you want more detailed advice, you can email or PM me privately.