at 3.5 more points are lost than won.
Very true about 'more points are lost than won' part.
Adding to this is the fact that every point is a slightly different situation.
At 40 love: It's ok to let loose on the first serve, but not the second...never. Believe it or not, but most of the time in this situation, the momentum is on your opponent's side. This means that you should be more aggressive, within your skill range (no miracle shots), but avoid getting down on yourself if you lose this point--most of the time you will.
At 40-15: The momentum has now shifted to your side, but not by a whole lot. Your opponent is probably thinking that he has escaped the wrath of losing the game in love; therefore, his mind is a bit less focused. He is now contemplating a come back (his goal is to get to deuce) and is probably thinking about the last point. Your job is to think about how you won the first three point and replicate that (easier to say than done, I know).
At 40-30: The momentum is now further on your side. If your opponent won the last point by hitting winner, he will want to try to replicate that. At 3-3.5 level, it is better to keep the ball in play (deep) here and entice your opponent to hit another winner. If you lost the last point (unforce error), you will want to be agressive here.
At deuce: In my opinion, unless your opponent has managed to blow you off the court in the last three points, the momentum is neutral. Your focus should still be on the first three points that you won and how you can replicate.
At his advantage: same strategy as at deuce. If he manages to hit five winners in a row, he is probably above 3.5 level and just toying with you or is having a really good day.
At your advantage: It's back to the 40-30 scenario, with a bit more momentum on your side, due to the fact that your opponent has gone through quite a bit to come back this far in the game.
I know, it's a lot of information. If you want a simple reminder, just think about the momentum at each point and focus on the first three points your won.