I would be very wary of offering anything 'up front'. Over what period would you wish to train for a PPL? If 3 of you stayed in the same bed-sit, studied together in the evenings, went out to the pub together now and again you'd probably help eachother as well as saving cash, perhaps?
From the flying school's point of view, they probably wouldn't be inclined to give you priority over other students unless you booked way ahead - and don't forget that no school is going to commit to a substantial discount too far in advance as who knows what will happen to the price of fuel, etc!
Nevertheless, if a group of 3 students wanted to book a couple of flights each per day for a 4-6 week period, I'd probably look favourably at a discount, although I wouldn't ask for anything 'up front' apart from the initial once-off joining fee. Don't ask though - I'm not advertising and we're booked solid right now and for the foreseeable....
[ 31 October 2001: Message edited by: BEagle ]