Mr. Faizal, yes you can not mix 2d and 3d element. if you are using 2D element to simulate the wall or other continuum and the interface between two materials or structure can be simulated by using the 2D contact element. Ex. 3-noded axisymmetric or 6-noded axisymmetric can be used to simulate the continuum and interface can be simulated using noded zero thickness or six noded zero thickness element. The interface for 2-D problem is to be connected by surface to surface contact but in case of 3-D is volume to volume contact. Therefore, i recommend to use three noded triangular element or six noded triangular element.