Why have both the 31mm & the 43mm?

Otherwise there is no reason to use Pentax at all - there is much better cameras available on market. Althoug all these three limiteds are gems!

From my experience I use 50% time 77mm limited

Sometimes 10% I use 31mm limited

And somsomsomtimes my wife uses 43mm (3% cases).

And something like 3% for each other limited or lens lens what I use (Da14 da15 fa28 fish10-17; fish sigma 15/2.8 fa 50/1.4 fa 85/1.4 etc...).

So, If you can afford (or afford in longer run) all three FF limiteds, buy them!