I have to throw in my vote for the old Lindberg kit. It's a shame no one has given us a good X-3 since the 1950s, when the kits were primitive to say the least. The Revell kit was originally issued in 1957 and as noted above is in 1/65 scale according to Schiffer's "Remembering Revell Model Kits."
The Lindberg kit is purportedly between 1/48 and 1/50 scale, and that seems about right. The shape is quite good, after comparing it to various drawings. The problems, of course, are that it's made of about 20 or fewer parts. The good news is that it is an almost perfect jumping-off point for detailing. That would include the undercarriage, making some sort of intake trunking and accurate exhaust. And a cockpit with an accurate seat, of course.
The X-3 was one of those many, many programs whose great promise was destroyed by the Westinghouse engine debacles, and though it looks like it's going Mach 2 sitting on the ramp, the only time it went Mach 1 was in a shallow dive. But it had a number of interesting, if ill-advised, innovations.
I have an article by a modeler who scratchbuilt the unusual downward firing ejection seat, which the pilot strapped on after it was lowered to the ground by a powered "elevator" system which had the lower hatch a few inches off the ground, and the seat on top of that, with rails that raised it up into position. Of course, the experiences with downward firing seats in the F-104A showed what a terrible idea that was in a single-seat fighter, and it claimed the life of famed test pilot Ivan Kinchloe and a number of other USAF pilots. Losing an engine on takeoff was almost always fatal.
Anyway, don't spend a fortune on a substandard Mach 2 kit when for less than ten bucks you can get the old Lindberg kit, modify the seat and install an instrument panel and consoles. You don't need much detail because from the top you can't see much at all. Also, some of those Lindberg kits come with blue-tinted windows. I'd use them for templates to make new and more accurate clear windowns, which are flat and easy to cut from card stock.
One more thing I remember: When Revell, in the early '80s I think, came out with its "History Makers" slew of reissues in fancy boxes at ridiculously high prices, they included the X-3 for about 22 times what it cost new. The series did not stay on shelves long after modelers figured out the scam.
TOM