Connecting 2D and 3D elements of a same structural element

I am trying this normal mode analysis for an assembly with 4 small bone kind of structures by welding. Since there is a porting of 1 mm thick I made it as 2D elements. But I couldn't get the connectivity between the 3D elements and 2D elements, which results frequencies of the order of 10^-3 and all.

I checked and done equivalence and all. But still its not working.

Can someone help me in solving this issue. Thanks in advance.

Note: I can't attach the file here, since size is limited. I am free to share the file.