I'm wanting move an event in 3D space. I know it can be done using Track Motion and '3D Source Alpha' moving the event on the xyz axis'.

My question is: Can this same motion be confined to one event among many on a track (as Pan and Crop can be), as opposed to having a dedicated track for a single event? If so how?

You can use 3D Source Alpha as Parent composition mode and move all tracks with Parent track motion.
(Page 386 in Vegas Pro 12 manual PDF)

You can do it using keyframes but it means more work for you.