I have only limited experience with my TS so far, but my hunch is that you could fit three under there but it might be a little cramped, especially if, or when, you have to pitch it low to the ground for more serious weather protection.
I used mine with a buddy this last weekend and there was more than enough room for just the two of us plus our gear and I plan to use it with an inner net tent for my girlfriend and dog at times.
The challenge with the Trailstar for three (as I see it) is that while there's a lot of covered area under it, the more usable area (for sitting up, etc.) is around the peak of the shelter.
Perhaps if you have long trekking poles and can set them up in an inverted "V" arrangement to do away with the middle pole or hang the shelter from an overhead branch (no poles at all!) or set the center pole off to one side at a bit of an angle, it would free up more of that central area under the peak for three.
You could always order it and try it out. From what I've seen, if it didn't work out and you ended up selling it, you'd probably have a line of folks beating down your door for it.
Hope that helps.