I don't know how to use "tm3d218.exe"? And when should I use it? Thank you1

That looks to me the 3D version of tmerge, a utility you use to merge different meshes into one. You don't access it directly, it is called by a program "utility".

Thank you! But, If I need merge different meshes, What should I do?

Open up a DOS windows and enter at the command prompt:

utility tmerge -3d -p sourcefilename1.msh sourcefilename2.msh targetfilename.msh

The -p means no other commands need to be executed, if you omit it the program will ask for translation and scaling factors etc.