I like the explanation by rishab1988. I'd go with too. "three years" is time, so it should be less than three years.
I guess the short of it is that units of time are not countable in the sense that apples, shoes, tires, and other visible discrete objects are.
rishab1988 wrote:*Less* is also used with numbers when they are on their own and with
expressions of measurement or time, e.g.:
His weight fell from 18 stone to *less* than 12.
Their marriage lasted *less* than two *years*.
Heath Square is *less* than four *miles* away from Dublin city centre
Numbers are countable right? You can say 1 ,2,3.But do you say 12 is fewer than 15?GMAT always says 12 is less than 15!
