The 3 is slightly closer to the side rail than the 7. The 7 is exactly even with the far point of the pocket. Trying to pocket the 3 in the side off the 7 will not work. At best, the 3 will hit the point and stay there.
Trying to get the cue ball behind the 8 is not as easy as some people seem to indicate, because you have to watch the angle you hit the 3 ball. You do not want to move one of his stripes therefore not leaving a combo, but a clear shot for the out.
Here is what I would do:
1) I would shoot the 3 ball 3 rails to come down where the 9 is, but I would shoot it a little long so that the 3 might hit the 9 off of the 3rd rail so as to tie up the corner pocket making it much harder for the opponent to get out.
I also would draw the cue ball on the shot back to the foot end rail where it would not be an easy back bank to the other corner just in case the 3 missed the 9. Leaving the cue ball on the foot end rail also leaves a real hard shot up table on either ball there.
This way I have played defense on all 3 balls of his, and the 3 maybe hitting the 9 ups my chances for controlling the table from there on out.