Anjil:
After hitting him with the 3 (which I understand you'd love to do), you would not be able to play the 5 anymore.
The official backgammon rule says "you have to play both dice if possible, and if you can play only either one, you have to play the higher one" - in this case, the 5.

The rules on this site also list this, towards the very end:
"If it is possible, both dice must be used. It means that some pieces can become "frozen" in certain positions because making a move with these pieces would create a situation where the second dice couldn't be used.
If only one die can be used, the one with the higher number must be chosen.'

Too bad, but according to the rule...