It is to 'ail'
If that could be "i," the answer is "ill."
Ill
ail example: What ails you?
I've is a word that means I have. I'll is a word that means I will.
Ick, ice, ire, ink, ill, ilk,
Bee, ill, all, tee, odd, add.
A 3-letter word that means "which person" is "who."
and
but
The 3-letter word for "sick" is "ill." If you take away one letter (the "i"), you are left with "ll," and by adding two letters (such as "a" and "s"), you can form the word "all."
Ado.
Let