Let's just say for the sake of telling a story that JJ is always +EV to 3bet preflop. (This is by no means a factual statement). This may be true, HOWEVER, it is certainly by no means the way which maximizes our expectation.

Whether a play is +EV or -EV itself isn't enough. In poker our goal is to maximize our expectation, and in the case of whether to 3bet JJ preflop or not, sometimes calling will maximize our expectation, and sometimes 3betting will maximize our expectation. The answer to which maximizes that expecation you seek lies in your opponents tendencies, their range, their perception of your range, etc. etc.

So pay attention and you'll figure out whether it's good or not.