I want to get tickets for a movie ahead of time to get a good seat. Example: a movie starts at 3 pm. Can I buy physical tickets for it 3hrs before?

Yes you can buy ahead of time and pick your seat from whats available. Also you can purchase seats online. Just browse to the theaters website.

Yes, you should be able to walk up and purchase a ticket from the box office 3 hours early.

You can buy tickets sometimes even a month in advance. I usually get mine a week or so in advance.

Yes, you can purchase e-tickets online.