All three have a trial version available. Adobe products are available for either the Mac or PC. Vegas runs only on a PC and Final Cut Pro X only runs on a Mac. I guess you could try Vegas on a hackintosh or Vegas on a Mac using something like Boot Camp and Windows. Final Cut Pro X is $299 (of course you would probably want to purchase Compressor for $49.99), Vegas Pro 12 starts at $599.99 and Adobe Premiere is available on a subscription basis only ($19.99 per month for Premiere Pro CC only and $49.99 per month for everything in the Creative Cloud). The CC subscription would give you access to Premiere, After Effects, Photoshop, Audition, and other Adobe products.
As far is which is best, well that all depends on the user. Get...