I can't say that I've ever seen or used "none the sadder" although I can't say there's anything wrong with it.
"None the wiser" and "none the poorer" are certainly used regularly.

Student or Learner
Dear all,
Do you use the phrase "none the sadder" as in the sentence below?
I found myself to be none the sadder for the next accident.
I have seen "none the wiser", but not "none the sadder" until recently.
Thank you!
OP
I can't say that I've ever seen or used "none the sadder" although I can't say there's anything wrong with it.
"None the wiser" and "none the poorer" are certainly used regularly.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
BNC has only these 4:
COCA has these 6:1 NONE THE WORSE 40
2 NONE THE WISER 26
3 NONE THE UNHAPPIER 1
4 NONE THE EASIER 1
So 'none the <comparative adjective>', except with a few words, isn't widely used. But 'none the worse' and 'none the wiser' are by far the commonest.1 NONE THE WORSE 69
2 NONE THE WISER 38
3 NONE THE RICHER 1
4 NONE THE POORER 1
5 NONE THE EASIER 1
6 NONE THE BETTER 1
b
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