Success! Our IT guy found this post 3.1.2 submission log errors and based on that it seems that the solution for this error of getting stuck on step 3 is to do the update process. Since we were on 3.1.2 and we were already thinking about updating to the latest 3.1.2-1 (a bug fix update, apparently) we decided to just go for it and hope for the best. By that I mean that we downloaded the latest 3.1.2-1 and then updated it through the Installed: Off flag as described in the normal update process, and now when I log in and go to the test submission I made which had previous stalled in step 3, now when I click Save and Continue it goes on to step 4 without any problems!