There are definitely things you can draw from the third PS game that you can use to make more informed conclusions.
For instance, if you see that your team is playing man coverage and your cornerbacks are getting roasted hard, that's a sign that your team is likely going to have issues in coverage.
If you have a coaching staff that does not change the schemes and limits the game planning, then I agree with you. I am not sure whether this is true of Coach Gase's coaching staff. The third preseason game is a process dress rehearsal not a an indicator of actual game planning. Just the process with still vanilla scheme.