How Do Adherent Cells In Culture Attach To A Plastic Dish?
I doubt that hemidesmosomes are much involved in 3T3 adhesion, just because they are characteristic of epithelial cells and fibroblasts are mesenchymal.

You're on the right track, though. It's going to be the same basic processes as adhesion to the extracellular matrix. The major factors will probably be integrins and focal adhesion assemblies.

You're also raising an important point for in vitro work, which is that adhesion to plastic is a very imperfect substitute for ECM, and this probably has implications for the behavior of the cells. If you're interested in ECM-dependent processes like migration, adhesion, and invasion, consider coating your plates with collagen or Matrigel.