Sanrensei simplest comments

Which side to block  

If White invades directly at 3-3, Black should definitely block as shown.

Continuation  

Black can be quite content with the standard sequence here. The stone is looking better and better placed.

Diagonal attachment

White approaches inside  

For White to play an approach inside the sanrensei is somewhat better than the immediate invasion (except in cases where White's take on the framework theory of the overall position is that making one group live quickly is a good idea). If there is a chance to play at a instead that is typically even better.

Black's diagonal attachment  

Now Black almost always plays the diagonal attachment, to set up an attack. This is the correct way to use . White's most reasonable continuations are at b and c, to form a small base.

erikpan: Quick question: why shouldn't black just take move 3 on b to prevent a base, and what can white do in that case? The two white stones look a bit lost after that.

Anonymous: The two white stones are already a bit lost; they're pincered by so a black stone at b isn't needed. If black does play 3 at b, it allows white to solidify his two stones by taking advantage of black's unfinished shape on the left.

Bob McGuigan: If Black plays at b then White can attach at as shown in this diagram:

White attaches  

This looks very good for White since his thick position negates Black's potential moyo, the reason the sanrensei was played. Also there remains some aji in Black's upper left corner.

The comment that is already pincering the two white stones is valid. Playing the "additional" pincer at is thus inefficient. However, in handicap games White might try extending underneath the stone as in:

White's slide  

This is not usually a good move for White because Black has various ways to take advantage of it, but it is also a place where Black can easily make a mistake.