Yes, I think it is a bad idea to put 3 tincts into a 75 vertical with only 10x10 of land. That's small for even one tinc in terms of floor space, let alone 3 which can cause problems since tincs are often very aggressive, especially with females. If you ditch the waterfall and pond, and keep it as a standard 75 gallon so there is plenty of floorspace the might be ok. And just to clarify, the citronellas would be kept separate from the oyapock, correct?
Bryan