Third person controller itself performs airbone or jump while steaping from one plane to another ?

Hi I am new to unity

I am working on a game where there are several plane side by side without any gap my third person character controller itself preforms airborne or falling down operation while steping at the joint of two planes . But works fine on the single plane or on a terrain

Can any one tell me how to solve this problem and let my player run or walk similarly as he runs and walks on the single plane

Thanks in advance.