(a)less(comparative of `little' usually used with mass nouns) a quantifier meaning not as great in amount or degree“of less importance”, “less time to spend with the family”, “a shower uses less water”, “less than three years old”
(s)less(nonstandard in some uses but often idiomatic with measure phrases) fewer“less than three weeks”, “no less than 50 people attended”, “in 25 words or less”
(r)less, to a lesser extentused to form the comparative of some adjectives and adverbs, indicates that the adjective or adverb is less of something“less interesting”, “less expensive”, “less quickly”