Are There any 3 Letters Left?

I'm just wondering because I'm using a GT checker and I'm not finding any available combinations. I know there are some for sale but i'm wondering if you can still change to one? If someone can reply with a valid answer I would appreciate it.

I believe they reset every so often, might not be correct, but that's what I got told.

If your checker is not getting any, then no, you will not get one unless you buy one or if one gets resets.