Some very basic remarks about sanrensei.
If White invades directly at 3-3, Black should definitely block as shown.
Black can be quite content with the standard sequence here. The stone is looking better and better placed.
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.
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.
See also: