Mounting Three Toons On Mammoth or Yak

I'm using a level 90 to power level two level 87 toons. I was having a problem losing follow on the last guy, so I decided to load them all on a 3-person mount. My main mounts, then targets himself, then I hit a wall. I have to dismount the vendors before I can mount the other two toons that I'm leveling, and I can't find a slash command to cover that, plus I have to assume both of those seats are different slots, like gear is configured. and while I'm asking for expertise on this subject, can you also tell me how to configure mounting my two toons after the vendors are removed? I'm pretty sure the last command will be IWT, do I need to retarget my main and add the assist before? The smarter I get, the dumber I feel