It depends on the size of the Discus... If you get 2.5-3in Discus they will be fine in that size tank for a while as Discus are slow growers...I would go with Four, even numbers do better for me or you will have an odd man out...
also it's typically a bag idea to have plecos with them right?
what about otos?
i have a 31" stand lying around and want to do something with it and 37's fit and i thought i would be a cool discus tank.
That is a good size tank to grow Discus. feed them 3-4 times a day and they will grow as fast as it takes you to save for a larger tank setup.
FYI I don't like to keep plecos w/ discus.