This is where I am, I need help to figure out if I am wrong; let me use Death by Thorns as my example. The way I read it, I could hit the same person with it three times to cause multiple saves by designating the same person as all three targets.
Why I think this.
Targeting: many spells produce multiple targeting possibilities, these can let you target all missiles / rays / etc. on one target, or spread them out to multiple targets in range. There does not seem to be anything that says you can't designate a single person as a target for a spell multiple times. Neither RAW, nor, seemingly, RAI. So you could, say, hit the same person with Water Walk multiple times, there just wouldn't be any point.
Layering Effect: Examples: Water Walk is pointless because any target you hit a second time already has the ability from the spell effect, though for what it is worth, they would then have to be dispelled twice. Ray of Enfeeblement (to use WoTC's example) can hit a target one round for a -4, then two rounds latter the caster tries for a better roll, but gets -3; the first one lowers the STR by more, but when it runs out, the other will still be in effect. Stacking effects rules are p171 of PHB.
Attack Limitations: You can make the three attack rolls even if you can't normally make three attacks, because the touches are part of the standard action of the spell spell. Reference hitting someone with Scorching Ray or the like.
So this would mean that if you cast DbT (BoVD for those who don't know) you could make three separate touch attacks, and any that hit would have their duration rolled separately, and the saves would be rolled separately. While the thorns were killing the person however, only the greatest of the d6's would actually deal damage (the lesser damages being overwritten by the more powerful spell effect).
But the point is that this would make it a SoD x3, with the caveats that you must hit with touch attacks, it offers a save, offers SR, doesn't kill them immediately, is a level 7 spell, and you have to deal with some small stat drain.
Am I wrong? Would you allow this if you were the DM?