For 3D use Blender. I think it is the only real alternative. BUT you will have to do a lot of work, because it is focused on animation. Blender is extremely wide, you will have to find what you need, what is not always easy for beginners. For example, there are plugin tools to improve exact 3D modelling, you might customize views etc.

For 2D use DraftSight (not free software). It is a clon of AutoCAD, which means that you don't have to get familiar with new commands, workflow etc. For individuals it is free, Information about your behavior will be send regularly to DraftSight. Go to their Homepage and get the actual .deb file. DraftSight's disadvantage is, that there is no 3D.