I actually emailed Proclip sometime ago and received this response:
Originally Posted by hrminer92
Thank you for your continued interest in our products. We will have to look into making a holder for this device. Due to an overwhelming request, worldwide, to develop all different kinds of holders, we must prioritize. At this time there are no plans to have a holder for the Pre 3. However, we do have three adjustable holders which are designed to accommodate a number of devices with or without cases/skins on them:
Small Adjustable Holder 511015
Adjustable Holder 875214
Large Adjustable Holder 848636
Each product page has the dimensions the holders will accommodate. Please take good measurements of your device with the case (if applicable) which will help you to select the best working adjustable holder for your device with the case.
Customer Service, ProClip USA, Inc."
Of course, because the Pre3's microUSB port is on its side, none of these will allow for charging unless modified. As an alternative, if your car's cigarette lighter can accommodate >1A, you can try to find somewhere to mount a spare touchstone...