Kind of irrelevant since OP is asking about S3s.

Currently the S3 is the best deal going.  If you pick up three, you'll need either a single PSU to drive all of them, or individual PSUs - one for each miner.  I use an EVGA 1300 G2 to power 3 units at a time.  That PSU can be had for about $150 on Amazon (after rebate).  I highly recommend it.

Now, please note that the 3 miners will draw about 1000W from the wall.  You need to factor that into your costs, unless of course you don't pay for electricity.  I see a mention of you going to college in a previous reply.  Make sure you understand the school's fair and acceptable usage policy for power.  Also, if you're going to have roommates, you might want to consider how they will react to 3 miners constantly running.  If you're going to school in a cold climate, they might be overjoyed at having 3 little space heaters in the dorm come winter time.  Of course, if you're headed to where it's warm...