Here are my thoughts while we wait on the experts to chime in.
Just in case, I would make sure your gravity reading was correct; for example, correcting hydrometer for temperature, stirred well.
Then you have several options. The best time to fix this is after the mash/before the boil. But it sounds like you maybe have already starting fermenting.
1. Just go with it: that's what I tend do do, especially if you've already started fermenting.
2. Add malt extract to make up the difference. For a triple, you probably want pilsner dry malt extract or liquid malt extract. DME has about 45 "gravity points" meaning 1 pound of DME will raise the specific gravity of 1 gallon of wort 1.045. So to go from 1.060 to 1.075, you need 15 gravity points per gallon. Assuming a 5 gallon batch, you need a total of 75 gravity points. That would be roughly 1.7 pounds of DME. Please check my math ... I've had a few pints.
LME has about 36 gravity points.
The easiest time to add this is in the boil. It wound like you are beyond that, so you will need to add the malt extract in jsut enough hot water to dissolve it. (I'm ignoring the effect of adding some volume with water in the above calculations). If you have already pitched yeast, you'll need to cool that enough to not kill the yeast and you need to be as sanitary as possible.
3. Add sugar. Table sugar has about 45 gravity points, so you would need about 1.6 pounds. This will "dry" the beer out a bit more than using malt extract. May not be an issue with a tripel, which are pretty dry anyway ... but that's quit a bit more sugar and I don't know for sure how that will go.