Oktoberfest

If you go in the afternoon, you should not need any reservations. Tables get reserved by large groups, many corporate for entertaining employees, customers, suppliers for the evening, starting around 6 pm. You can sit there at least until the time marked on the reservation card, and sometimes later if everyone doesn't show up. Talk to the group organizer, who should be obvious.
As for as which tent, the one whose beer you want to drink.

Its like a massive church picnic: folding tables and chairs - not like restaurant seating. I don't think you can book a reservation for three, and its not like they have hosts seating you. Check out a few of the tents and pick the one with the atmosphere that appeals to you.

It's only possible to get a reservation for three by going through a travel agent or reseller as the tents only sell reservation for entire tables. The good news is with just three people on a weekday you should have no problem finding a spot in the afternoon.

DJ

I have found Tuesday and Wednesday's to be the least crowded.That being said get there at 10am if you want to walk through a few tents ,last time I went they started reserving tables for 4pm ,if you're sitting at the table the make you leave.The Hacker-Festzelt followed by the Schottenhamel tent are awesome.

You wont need reservations and almost certainly its not an option for you anyways. 3 is an easy crowd, although is that the first Monday so it might be a little more crowded, ust show up early.

Hacker Festzelt is the place to be.

Thank you to all for your wonderful responses. We are looking forward to our first experience with Oktoberfest. We have been to Munich a few times but this is our first trip during the festival.