When you get a 3 more rep you have to hit the draw button and claim the draw. It won't do it for you automatically.

The draw button changes from 'offer draw' to 'claim draw' on repetition of moves, but you have to click on the button to claim the draw, as zxb995511 said.

It might be an idea for chess.com to have the label on the button change colour when it changes from 'offer draw'. A player offered me a draw in a game some time ago, and I think in this case the button changes to 'Draw ?'. The problem was, it was several moves before I noticed the label change, and accepted the draw. My opponent probably thought I was being stubborn.