Hi and Welcome, Can you please post the drawing or a sample drawing, also what version TC are you using and is it pro / deluxe
RE - I was able to make a drawing using drafting pallete but i get solid blocks in the drawing (Instead of 2D sketch with only outline)
I assume you tried the other icons in the drafting palette which give different types of views, normally the fill (solid or hatched) can be removed by exploding the drafting palette view once its in the drawing, and then selecting the solid / hatch and deleting it.
Re - Also one of the obj files is a combination of 3 models. How can I delete parts of the model (unmerge and delete parts)
Need to see an example, but it would depend on what TC's import filter made of the objects, it may have combined them in which case the one way would be to explode a drafting view and delete what is not required, if TC simply grouped them then you could explode once to get the objects, tough to say without an example.