I'd second the late addition of a good portion of the extract. I do all partial boil brews and I do all my hop additions (except flameout) with just the runnings from the grain then add the extract about 5 minutes before boil is done. Extract has already been boiled when it's made, so you pretty much just have to heat it up to sanitize it and get it to incorporate properly. Since that kit uses a good amount of DME and a smaller amount of LME, you might start with the LME at the beginning and then add the DME towards the end.
That and butterpants' other suggestions are all great for pretty much any brew, not just for a tripel. That kit also includes a candi syrup. I'd save that and add it directly to the fermenter just as the ferment has peaked and is beginning to wind down. That'll give the yeast a chance to work through the maltos before giving it the simple sugars.