EdFred
Taxi to Parking
For those three letters, all of the airports I've landed at have numbers in 'em. K52, X39, Y50, etc. So, you might not have to go too far - There are some Y## ones in WI. The K's are in (and presumably around) Missouri. X39 is North Tampa so that might be a good spring break trip to collect some X's.
Something that I learned a couple of years ago: Most of the ID's that are one letter and two numbers, the letters denote what ARTCC's airspace you're in. For example, there are LOTS of airports around here that are C##, #C#, or ##C - All in Chicago Center airspace. Y is Minneapolis Center, I think K is Kansas City, etc. I believe some centers have 2 letters.
I want all letters I already have K58, Y83 (among others) Z98...which makes K the most elusivse in the CONUS.
KIC King City, CA
KLS Kelso, WA
KNB Kanab, UT