Well, since triples usually aren't dry hopped I would say this is really up to you. By dry hopping you are going to get a more aggressive hop aroma so it's really how you want your beer to smell.
Some things to consider, if you are using a lower alpha acid hop like a noble hop you will introducing more vegetal matter into your beer which could lend to a more tannic unpleasantly bitter quality. If using a higher alpha acid type hop it might be too aggressive to blend well with your beer.
I would start with a 1/2 oz for the lower alpha acid and 1/4 for the higher alpha hops. If your beer does become too tannic just give it a couple of weeks as this subsides with time.
You can find hop charts in many places online to help you make your choice. Keep in mind though, that once you dry hop your beer it will no longer be a triple if you plan on entering into any competitions.
