Hi Jack,
As Ian said, welcome to the Cayenne community!
Hmmm, what makes you say its because of python 3.4.2?
As Ian said, we don’t officially support Jessie Lite, although there is a work around here.
Please also make sure you expand your file system prior to installing Cayenne onto your Pi.
I also recommend updating / upgrading the Pi 3 as well.
sudo apt-get update
sudo apt-get upgrade
Please keep us updated with your progress!
-B