If you're new to triangulation, understand that you don't confirm the MRCAs with that, the exception being if you're triangulating with a half-relative in the mix. Unless that's the case, you don't know which ancestor in the pair contributed the segment.
For gt-gt grandparents, it's easiest, usually, to find a suitable 3rd cousin (or a 2C1R in you parents' generation) and confirm them without triangulation. With that kind of confirmation, you get to mark the MRCAs.
So to confirm a gt-gt-gt grandparent, you need to triangulate with people who have your corresponding 4th-gt grandparents as the MRCA.
So it's a mess. The places you can compare chromosomes often don't have the matches, and the place with the most matches doesn't offer chromosome-level comparisons, and it's hard to get people to answer a message, much less upload elsewhere. It seems almost like some kind of conspiracy! 
That being said, even when triangulation doesn't help confirm an ancestor, it can still be used to confirm various family lines coming down from those ancestors, and even if you can't do THAT with it, it give you more confidence in knowing where a segment came from, if you're mapping out your chromosomes.
Be aware, also, that if you can find someone in your parent's generation who has a test, who is also descended from you gt-gt grandparents, and then find a 3C of THEIRS, that you can use THAT match (a match that does not include you) to confirm those gt-gt-gt grandparents, without triangulation. I've done a couple of those, myself.
Good luck!
