How you want that? Thresh, Rakan and Bard are already high impact supports, where skill rewards you much more as on the other Supports. You know, a Janna only can press her shield and Ult adn q at the right time, but to use thresh e the right way, to hit the hook, to decide wheter to recast it and go in or not, with maybe a lantern followup. Or when to use the Bardult, when to go in with Rakan Combo, these are just other levels as playing tham kench, Braum, Soraka, Lulu etc. Yes, the 3 are no full tanks adn no full entchanters either, they are designed as hybrids so they need to build kinda hybrid.
Entchanters and Tanksupports need the items to support farther for their lack of impact compared to high skill Supports. High skill supports naturaly that they are not full tanks or full entchanters don't profit that much from items but in exchange can carry harder with their abilitys in a fair meta.
One item, could be good on Thresh and Rakan, would be
: it doesn't favor tanks realy more, farther it favors champions who go in, siad champions or Leona, Alistar, mostly Blitzcrank probably (who carries with his high impact q).
So Items that favours going in could help thresh and Rakan while not so much Bard, maybe an Item that favors hitting Spells or Ults on enemies? idk if this is realy needed and farther that could easily be abused by other supports or even other classes as well.
So yea, to have an item supporting the most hese 3 champions is realy hard, especialy cause they already support good when skillful players play them.