Respectfully, I disagree. 3d apps are fine, and they are powerful, but they're also quite complex. If you're just making some isometric icons or somesuch, taking the time to learn Blender or Daz, or even Sketchup, may not be the best use of time.

If you are going to dabble in 3d, then Sketchup is a good place to start. The interface is, in my opinion, much more intuitive than any of the other free 3d apps available.