As you can see from my signature, I managed to get Firmware v7.903 working on my One V3, thanks to 46Jimbo for helping me with that!
How you get the firmware is upto you.
It wasn't too much trouble once I got the firmware, but you can't use TT Home to do it. You have to copy all the firmware files to the Tomtom using Windows Explorer. Of course, you should make a backup of EVERYTHING in your Tomtom first and put the backup somewhere else on your PC. This is all at YOUR OWN RISK. Once you've backed everything up just delete everything except for the map directory from the Tomtom (the map directory will be pretty obvious). After that, just copy everything over from the firmware directory into the TomTom and then disconnect the TomTom and switch off and on again. That's it!
If you had any custom programs, such as Suntime or MenuTT or custom menus, you can just copy all that stuff back to the TomTom. This worked fine for me. I had trouble getting the custom menu to work again, but I think I know why.
At this time, I haven't tried any new v8.xxx map on my TomTom since the firmware upgrade, simply because I'm waiting until I know whether TomTom are going to bring out any new maps between now and the end of March.
Hopefully this will help anyone who has a V3 and wants to run the newer maps on it but keep the Itinerary planning feature!