I'm going to steel the diagram from dfan.

to give my view on this.
$$Wm1
$$ +--------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . 1 . . . . . . . . O O X O .|
$$ | . . . X . . . . . 3 . . . O X X O O .|
$$ | . . . . . . . . . . . . . O X O X X .|
$$ | . . 2 . . . . . . . . . . . X . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . X . . .|
$$ | . . . , . . . . . , . . . . . , . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . X . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . X . X . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . X . O . . . , . . . O . O . . .|
$$ | . . . . X O . . . O . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ +--------------------------------------+
- Click Here To Show Diagram Code
[go]$$Wm1
$$ +--------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . 1 . . . . . . . . O O X O .|
$$ | . . . X . . . . . 3 . . . O X X O O .|
$$ | . . . . . . . . . . . . . O X O X X .|
$$ | . . 2 . . . . . . . . . . . X . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . X . . .|
$$ | . . . , . . . . . , . . . . . , . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . X . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . X . X . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . X . O . . . , . . . O . O . . .|
$$ | . . . . X O . . . O . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ | . . . . . . . . . . . . . . . . . . .|
$$ +--------------------------------------+[/go]
the proverb "building territory with you thickness is bad"
This would apply if you played 3 first, building territory with you "wall" therefore playing 3 first is bad according to the proverb and I agree with that. It allows black to play at 1. this takes away any further development of the top for white, while black has just gotten a nice move in the corner to turn the left in a lot of potential for black.
The top left corner approach at 1 looks like a good move. It is far from the wall, so rather than building points with the wall you are using it to create potential (a good thing).
If your opponent plays 2 you can play 3 (like in the diagram). now 3 is a good move, because first and foremost 3 is there as a follow-up on the 1 stone, secondly as an EXTRA it is creating territory with the wall.
this makes the move at 3 a dual purpose move and therefore a strong move.
if your opponent resists like dfan also showed then black gets a weak group and you can expect some attack later.
side-note
The top right is indeed weak and the cutting-points really are a problem, (but that was not part of your question).