finding columns that have three or fewer distinct characters

I've got an interesting one...I'm trying to find columns that have three or
fewer distinct characters (for example, "aaaaaaaaaa"). Wondering if I need to
write a function or if someone has an idea how to do it with built in
functions and/or pattern matching?

I think the thing to do would be to lowercase everything, then remove all
duplicate chars and spaces, then use length() on that, but it's not obvious to
me how I might remove the duplicate chars with the pattern matching support
in the docs.

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Jeff Frost, Owner <jeff(at)frostconsultingllc(dot)com>
Frost Consulting, LLC http://www.frostconsultingllc.com/
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