You know, I tried loading the hi toms sfz file into one of the 4 pads and got no sound.  When I looked in the help, it mentioned that the sounds have to match the category or you won't hear anything, which sounded like what I experienced.  Apparently, you have to load toms into one of the "toms channels" and not a "pad channel", which of course makes no sense at all to me - what does Session Drummer care what instruments I load into a channel?

The sfz files contain information about the samples and what notes trigger them (and a bunch of other control information).
 
In Session Drummer 3, the notes are hardwired to the pads. This is why loading an existing "hi toms sfz" makes no sound when loaded into velocity pad 10. The "hi toms.sfz" contains sample information mapped to notes 48 and 50 but velocity pad 10 only works with notes 39, 56, 58, 69, 70 and 75. To get velocity pad 10 to trigger the "hi toms.sfz", make a copy of the file and change the "key=48" and/or "key=50" instructions in the new sfz file to the notes used by velocity pad 10 and load the new sfz file into velocity pad 10.

Excellent!  I have tweaked a couple of sfz files to customize kits with my own samples, but I didn't know I could trick the pad into loading other drums.  So, now a have a couple of options to try.  I'll post again when I've tried them and see what works best.

I think there's some confusion about channel numbering here.
 
SD3's 12 outputs correspond to 24 mono channels of audio in 12 stereo pairs. When you look at the list of available outputs from SD3 in the Input picker for an audio track you'll see a left/right/stereo pair for each of SD3's 12 outputs. But the numbering doesn't correspond, so Out 1 in SD routes to track inputs 1-2, Out 2 routes to 3-4, Out 3 routes to 4-5, etc.
 
So there are two ways to pan the three Tom pads independently:
 
1. Assign all three Tom pads to the same stereo output pair, set up a single "Toms" track with Input set to that stereo pair, and pan the toms in SD3's mixer as desired.
 
2. Assign each Tom pad to a separate stereo output pair (leaving their pans centered in SD3's mixer), set up three mono tracks to receive only the left or right output of each pair (doesn't matter which), and pan the mono tracks as desired.
 

brundlefly, thanks for the input (no pun intended).  I do get the channel numbering/output scheme, but scook understood the nature of the problem and offered a couple of suggestions.  The crux of the problem is that there is no way to get the two toms (e.g. notes #48 and #50 hardwired to Session Drummer input channel 4) routed to two separate outputs.  No matter which of the 24 outputs (or two if using stereo) you chose to route input channel 4 to, both toms will always be routed through that single pan control - there's no way around it.
 
I think the design limitation in Session Drummer is that there are only 12 inputs and each of those is further restricted to only certain midi notes (e.g. #48 and #50 for input channel 4).
 
I was able to load modifed sfz files into the pad channels 9 through 12, which worked okay, but I quickly discovered that I was going to run out of channels for big rock kits with 6 or more toms and 3 or 4 or more cymbals.  It also would require copying and editing every sfz file for every different kit I wanted to use etc.  Ugh.  That quickly led me to drum maps which turned out to be a much more viable solution once i figured those out.
 
So that's where I am now, although I've run into a few glitches with inserting from the track template I created for this setup.  The audio drum tracks that are routed directly to my output buses (17-20 for K, Sn, Hat) get reassigned to 1/2, while the rest of the drum tracks that are assigned to stereo buses (Toms and OH) get assigned correctly, but the two stereo buses themselves get reassigned to 1/2.  I also have to recall my drum map separately for each project and assign that to me midi drum track.  So there's some manual labor involved.  It all works fine when using a new project template, but inserting from a track template into an existing project is not so reliable.
 
X3 may not have these issues, but I'm still on X1 because I'm using a Motu 2408 w/ a 324 card in an XP box, which doesn't have 64 bit drivers.  My system's quite stable so until I upgrade my card, I guess I get what I've got.

Make sure to save the SONAR buses with the tracks in the track template. I always assign all tracks to buses in SONAR. Only SONAR buses make their way to interface outputs.
 
You are really pushing the envelop with Session Drummer but it will work up to the physical limits of the hardware. Likely to have RAM issues with very large kits in 32bit hosts and not much can be done about it with Session Drummer, everything loads in RAM. You may have an easier time with top of the line drum synths like BFD (although BFD3 does not support XP) or Superior Drummer (which still supports XP) because they have ways to run larger kits is less RAM. They also have ways to build larger kits but still would require some sweat equity to create very large custom kits.
 

bobmc
The crux of the problem is that there is no way to get the two toms (e.g. notes #48 and #50 hardwired to Session Drummer input channel 4) routed to two separate outputs.

I guess it depends on what kit you have loaded (or if you're building your own SFZs), but as tomixornot pointed out in the first response, at least some if not most or all of the bundled kits have the same sample mapped to both note numbers for each Tom so there's really no need to pan them differently.
 
In any case, I have to say that I don't feel Session Drummer is worth getting too carried away with. It's pretty quickly outclassed by many relatively inexpensive drum synths in terms of features like mic positioning, top/bottom and in/out micing, room and overhead signals, built-in processing, bleed/resonance controls and overall sample quality, variety of articulations and number of velocity layers provided in the bundled kits. If you're really that into tweaking drums, I think your time might be better spent with some of the more state-of-the-art 3rd-party offerings.