Why "three student" is not a minor slip

Saying "three student" instead of "three students" is not a harmless colloquialism; it signals to an educated reader that countability has not been internalized. In English, every count noun after a number above one must carry the plural marker, and omitting it changes the noun's grammatical identity. A quick mental check -- find the noun, confirm it is countable, then match it to the number -- fixes this error permanently. The practical camp may argue that the meaning is still clear, but clarity is not the same as precision, and in formal writing the reader expects both.