Tribhuvandas did believe Kareena @ first. But u guys have forgotten he doesn't know Lolo and Mom were hired by Tushar to stop his marriage with Kareena. Lolo and Mom told Tribhu that Kareena paid them to say no to Tushar, and thus leading to Tushar's accident. Lolo and Mom also found out that Tribhu was on Kareena's side so they became clever and told him that she was using him to get to the Chamatkar wealth. Tribhu believes that Tushar and Lolo love eachother, and Kareena is after Tushar even though she knows Tushar doesn't love Kareena but Lolo.
At first i 2 was angry at Tribhu, but then they slowely reveal what happened, you have to listen very carefully to hear what Lolo and Mom did, its broken into a series of parts and conversations that aren't given very much importance.