RUE
regret
MIRROR is a six letter word with three r's in it
The five-letter word with three 'R's is "error." It fits the criteria by having three occurrences of the letter 'R' and consists of a total of five letters.
The three-letter word for scull is "oar."
ore
rat
E A R
mar
air
tar
Mirror has three rs and its a 6 letter word. so it has the most percentage of R's. Is that what you mean?
A three-letter word that means "exist" is almost always "are." See if it will work.