If you're playing leader in T3, chances are you're playing a mono hand. With limited choices for mono cards, it's easier to examine more precisely, what are the strongest combos of leader and clan?
For me I would say, the most obvious is Hugo + Montana or All Stars. Because Hugo pushes you over the 8 min limit so no pill bluffs become a massive +19 attack virtually.
However, this combo is completely derailed by Raptors. And the high 9 powered Raptors like Impera Sloane or Scarlacc shred Montana.
Stronger is the All Stars Hugo combo. Power reduction + attack manip is a legendary combo. B Mappe Cr is an extremely shaky card on its own but Hugo perhaps does more for B Mappe Cr than any other card in the game. Against flat 8 powered 5*s, a one pilled Mappe will always win. Roderick is almost unbeatable, going from 12 attack and -2 power to possibly 27 attack and -2 power. 5 pillz and flat 8 powered cards must use all pillz to win. Other cards like Zlabal, El Gringo, Sigurd or Lamar Cr become far stronger with Hugo's presence.
In the same vein, Eyrik does well with all attack manipulators. With Mona Cr he can reduce 7 powers to 4 and with 1 pillz, 5* need 7 pillz to win.
Bridget is slightly decent with Huracan but overstated, 1 life doesn't do a whole lot and at max its +3 more attack. Better for Huracan is Eklore. Eklore encourages your opponent into pilling big in early rounds where you can therefore pill big to match, try to win and secure your bonus at max power.
Bridget is actually good, however, with Frozn. Frozn is scary to play in T3 and almost impossible mono because activating your bonus twice is not only predictable and negated by tons of DR, SOA, SOB or damage exchange, but can bring you close or to death. Bridget can make sure you stay above 0 health while turning revenge on. Better than Bridget is Vholt with its mighty -2 DR.
Ambre pairs well with Pussycats because like Pussycats, Ambre is give and take. Lose one round to overwhelmingly win more important rounds, and lose safely to create an uncollapsible gap. + Pussycats makes Ambre less one dimensional. Is Charlie or Missandei being played to defend? Regardless, it's difficult to deal damage and even more difficult to win.