The recipe scales exactly linearly. Just multiply all the ingredients by 3.
For figuring out how much yeast you need you should use the Mr Malty Pitching Rate Calculator
to figure out how much yeast you should be pitching.
15 gallons of 1.094 needs a seriously
hefty amount of yeast, and this is an absolutely crucial step you should not skip.
Without a starter you are looking at 11 vials of yeast from whitelabs/wyeast. If you do a simple starter (no o2, shaking or stirplate) you will need about 2 vials and a 32 liter starter. If you have a stirplate (your buddy should REALLY have a stir plate with these big batches) you can get by with 2 vials and a 12 liter starter.