It varies depending on the river that you are fishing. For most medium sized to large rivers - a distance of about 3ft between dry fly and nymph will be about right. This allows the dry fly to "lead" (i.e. travel ahead of) the nymph. This provides the take detection. Match the weight of the nymph to the depth of the water so that it hits fishing depth within about 2-3 seconds of landing. This means that you can keep your casts short and hold all of your fly line and leader off the water right up to the dry fly.
When I use a trio setup a usually have about 4 feet between dry fly and point fly. I vary the position of the dropper depending on whether it carries a soft hackled spider pattern (12 - 18" below the dry) or a small pheasant tail/hare's ear nymph with a 2mm tungten bead (24 - 30" below the dry). This tends to match the feeding depth of fish for drowned adults/failed emergers (spider pattern) or pre-emergent nymphs (small bead head).
In very tiny rivers or fishing in shallow runs - it might be a duo setup with as little as 12 - 18" from dryfly to point fly.
Again - I'll plug my DVD that covers these setups as well as reading the water and controlling the drift etc. Its home made but has received 100% positive feedback from all purchases. PM me for details and see here for some comments:
River fishing DVD