The beers probably still good. And a longer (3 to 4 weeks) is better than a short fermentation (1 to 2 weeks). But there is a point when the beer is done and stops aging. You reached that point months ago. You beer is still good and is probably better than it would have been if you brewed bottled and drank within 4 weeks. But it can't be honestly said that leaving it for three months will make "better" beer.
not a good generalization. best to say "depends on the type of beer". a hefe is better at 10-11 days ferm than 25 days. a blonde doesn't need to go that long, either. in-your-face hoppy beers, not so much either. i started moving toward the idea of 10 days ferm, then cold-crash @ 30-32 ( on the advice of belgian brewers in "Brew Like A Monk" as well as our own Yooper ), and my beers went up a couple notches
Remember, yeast determine fermentation time, not brewers. We can influence it by our ingredient choices, quality control and temperature, but they are finished when they are finished. Aging is an other matter. Some styles benefit, others simply fade. Either way, assuming the beer was good in the first place, 3 months in a carboy shouldn't hurt a thing, and may have even made it better. Take a sample and give it a taste; what do you have to lose?
Depending on the style, it may be faded past it's prime, especially for a hoppy or yeasty beer, but it should definitely be drinkable. Lagers, Trappists, Stouts, Porters, many styles can age beautifully.
Best bet is to try some, unless it was infected, it may be great. Even if infected, it still could be great, just different than expected.