Sanrensei simplest comments
Some very basic remarks about sanrensei.
3-3 invasion
Black can be quite content with the standard sequence here. The stone is looking better and better placed.
Diagonal attachment
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.
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.
See also: