Also, remember that a company will fire you in a minute if they no longer need you or can't afford to pay you. You shouldn't feel the need to have more loyalty to this company than they have to you (which is zero).
They _can_ get rid of you immediately but it’s not customary.
Most employers who want to maintain any sort of reputation will provide severance. That is the other side of the same coin as giving advanced notice of leaving.
That said normal notice is counted in weeks not months.
I would assume/hope this means you have a senior role and are well paid. This is not uncommon for members of a leadership team etc to ensure reasonable transition times and stability. Also most employers will show flexibility here in my experience. E.g. A couple of jobs back an employee just requested I be on call for a couple months for the new person should they need help and kept paying my salary just to be available.
That said, if you're a lowly paid shit kicker that would seem wrong as typically a new junior job they want to to start ASAP and wouldn't wait for 3 months. So that could hurt your ability to find new work.
I'm not in a senior role. I do enjoy my job but I'm basically a sole Rails dev - I develop an app that the company offers to clients, which I suspect plays into the long notice period - I suspect they'd have a fair amount of difficulty replacing me.
But yes I suspect that's part of why they have such a long notice period - when I've spoken to recruiters / hiring managers elsewhere they're interested right until the point where they discover I have a three month notice period.
Then I'd suggest ensure you have some savings together to cover a 'just in case' prolonged search then talk to your employer that you need a better notice period as you keen for something new. Pitch it as this gives to time to find the right job rather than rush off. They might be reasonable. You might even end up with a counter offer that makes you want to stay.
If they won't change, quit on the spot. Then you've got 3 months to find a job.
You could in theory have an employment contract that requires notice. But it probably wouldn't have much force; such a contract would establish that failing to give notice would be a breach of contract, and you could theoretically be on the hook for making your employer whole for the "harm" you caused, but unless you were given some significant consideration for your side of the contract --- a signing bonus or something, maybe? --- the harm your employer could demonstrate in court would be so minimal that the cost of enforcing the contract would far outweigh the return they'd get from enforcing it.†
In practice: a pretty rare situation, and not a meaningful deterrent to quitting at will.
I believe the cases where you're meaningfully prevented from quitting without giving notice, generally, are cases where you already know clearly that you can't, and why you can't, because you negotiated your contract and know what you'd be giving up to leave. There are key-person agreements that get signed in acquisitions that have that kind of effect.
Employees, though? Not so much. Employers in the US that purport to require notice from rank and file are pretty much generally full of shit.
(Not a lawyer, have just seen a lot of weird employment situations).
† Signing bonus claw-backs are also super common outside of "notice" situations, so it's not like that's an exotic case; if you have a signing or retention bonus, you probably already know you have a contract requiring you to return it if you leave within N months.
Do you work in the U.S. as an "at-will"[1] employee? If so, your employer can't "require" anything of the sort.
The vast majority of employees in the U.S. fall into this category, and are free to leave their jobs at any time - and their companies are free to fire them at any time. Under this system, giving notice of your resignation is a courtesy, not a legal obligation.
Are you working somewhere abroad where this is a norm? It is extremely unlikely that's a reasonable demand in the US. You can quit on no notice in the US.
