From laptop to Zumo (I have the XT2 but it is the same as I've read the manual), there are some options.
Use the connector direct from MRA as mentioned above. That is a USB cable requirement.
Save the route as a GPX 1.1 (route, track, POI) to your laptop hard drive. Copy and paste this route to a GPX folder on your Zumo. My strong suggestion would be to insert an SD card and create the GPX folder on there. That way, the Zumo doesn't get to mess with it until you import it and you get to delete and re-import as many times as you like. If it's on the device itself, once you delete, it's gone.
This is also a USB cable requirement obviously.
Tread app. Download the MRA route (from the MRA app on your phone) to your phone files. The process is this: 'Routes & Tracklogs - open route - scroll along the very bottom line and click the down arrow - select gpx1.1 (route, track POI) - Download. Your phone will then give you the option of where to send it. Pick Tread. Then the fun begins. If you've built a good route, Tread should replicate it and it will sync to the XT3 if you're all connected up properly.
This is the wireless way so can be used when out and about with no laptop.
There are plenty of help options for if any of this goes wrong. On here and Zumo User Forums. Not much on there about XT3 yet but you can find all this on the XT2 pages as very similar.
I may have missed something out slightly but I think that's about right. Happy messing about.