First, I did a search on Google images for gift enclosure cards. There are quite a few ideas there. Here is the link:
Next I just sketched out a 8-1/2 x 11 sheet. If you make your cards 2-3/4 x 2-3/4, and cut them like my sketch, you can get 6 of them from a sheet of 8-1/2 x 11. The solid lines on my sketch are to cut, and the dotted lines are to fold.
This will leave 1/4" left over on the 8-1/2 width.
PS click on the thumbnail sketch to make it larger.