Thomas algorithm tridiagonal matrix

Hi,

how to solve a tridiagonal system by the thomas algorithm with 3 equations (each equation has a tridiagonal system) .. the first equation and the second equation depend on the third equation (coupled system).

Knowing that Thomas algoritm is based on two steps: (forward elimination and backward substitution), How can i solve the 3 equation one after the other in the same iteration... (because they are coupled)?.