A lot of the answer to that really depends on your physical conditioning, and that's really an answer that no one can supply definitively. After three months you'll definitely notice that you can't do something as well as you could when you were "in shape", but it may be as simple as 1kph on an average, or you can't climb that hill at 20kph that you could before. Unfortunately in any sport - and cycling is certainly affected there's a certain amount of conditioning that's lost with time away.

I know for me I'd be really fresh at first, but after a couple days it would be a noticeable decrease as my endurance suffers. I took vacation in early December and just went back on Tuesday, and my performance has noticeably decreased. That said, it's also 20 degrees out, and I'm riding a (new) touring frame I built over the vacation. All may be factors. I'm also a 54yo diabetic, so my physical conditioning isn't the same as a younger person as most are undeniably in better shape on a bad day than I am on a good one.

As a counterpoint - my son who is 19 and was fast enough to qualify for USAT Team USA stepped away from the bike for 4 months, and decided to join a fast group ride in the mountains here over the holidays. We had to pick him up with the car at mile 20.. so it's not just me

I'd probably try what you are intending, but I'd have a "Plan B" should it not fly as Juha has suggested.