The short answer is "Yes."
People in your generation who are descended from gt-gt grandma's siblings are your 4th cousins, so you would need to triangulate with them (since they're more distant than 3C). If they're in the generation above you, then they're still 3C1R and it's still no good.
HOWEVER!...
(1) You have to keep in mind what your goal is with this. Triangulating these people will NOT DNA confirm you any further back in your pedigree, because you don't mark the MRCAs with triangulation, the way you do with 3Cs..What it WILL do, however, is DNA confirm some of those family lines, going down from those aforementioned siblings, down to the people you match. So if THAT's your goal, go ahead and start begging folks to upload to GEDmatch or some other place where you can triangulate.
To DNA confirm further back with triangulation, you'd need at last 5th cousins.
(2) If one of those matches is in your grandparents' generation, and it is strong enough, that'd be a 2C2R, which I'd argue is equivalent to a 3C, and you might be able to actually use it.
(3) A better route, however, is if there is a match on your line in your parents' generation, and also one from that generation on another line. THEY are 3C, so if they will tell you about THEIR match to each other, maybe you can use THAT to go back a generation. If their match isn't strong enough, you could still use it without breaking the rules if you can get both parties on GEDmatch (where there's no such thing as a "predicted 3rd cousin" match). I have actually used this, myself, in a place or two.
