I suspect the OP may have made some typos as I'm not sure what a "Highway K Frame" is nor have I ever seen, or even heard of, a modern 3-shot revolver. I don't think it would be possible to safely convert a K-Frame to .50 AE regardless of how many, or how few, shots the cylinder holds. I just measured the outside diameter of the forcing cone on my M10-5 and it's right at 0.500". Given that the bullet diameter of .50 AE is, depending on the source, either .499" or .500", you've got zero room to increase the bore diameter unless you increase the OD of the forcing cone and thus remove material from the frame.
I suspect that the .45 LC K-Frame that Jim Watson mentioned likely had very thin forcing cone walls and probably couldn't handle ammunition with pressures higher than the 14,000 psi SAAMI limit for .45 LC. I know that the Taurus models 445, 450, and 455 which were chambered in .44 Special, .45 LC, and .45 ACP respectively and were roughly K-Frame sized all had paper-thin forcing cones and could not handle hot-rodded ammunition. Given that .50 AE's max SAAMI pressure is 36,000 psi, the same as that of .41 and .44 Magnum, I don't see how it could be made to work safely in a K-Frame as there just isn't enough material there.
Like I said, I don't know what a "Highway K Frame" is. Did the OP perhaps mean a Highway Patrolman (aka M28)? This would make more sense as the Highway Patrolman, while chambered for .357 Magnum, is built on the much larger N-Frame and thus has room for a much thicker forcing cone. However, even in an N-Frame I think .50 AE might be stretching the limit as you've got a significantly larger diameter bullet than anything the N-Frames have traditionally been chambered for (biggest diameter I've ever heard of S&W making in an N-Frame was .455 Webley) operating at the same pressure as the highest pressure cartridges for which the N-Frame is chambered (.41 and .44 Magnum). Conventional wisdom for a .45 LC N-Frame like a M25 is that, while they can safely exceed the 14,000 psi SAAMI limit, it's best to stick with "Tier 2" loads which max out at 23,000 psi (same as .45 ACP +P).
The only DA revolver I've ever heard of being converted to .50 AE is Hamilton Bowen's "M1917" Ruger Redhawk which is a 5-Shot .50 AE that uses moon clips. Of course, a Ruger Redhawk is larger still than a S&W N-Frame and capable of handling 40,000+ psi pressures. Of course, it could be done easily in a S&W X-Frame as it is chambered for .350 Legend, .500 S&W Magnum, and .460 S&W Magnum which have max pressures of 55,000, 60,000, and 65,000 psi respectively, but doing one in .50 AE would seem like a step backwards when they're already made in much more powerful calibers.