Linux lite is just an adoption of the Ubuntu long-time stable releases.
The oldest still supported of these is Ubuntu 18.04, which already shipped with cmake 3.10. I think Linux lite v3 is based on the now-defunct Ubuntu 16.04.
So, your Linux lite must be very very ancient, and you will very likely encounter much more missing components even if you managed to Backport cmake to it.
It is time to update your Linux distribution. (That is definitely the easiest and least work way of getting cmake >= 3.9, so this is the answer to your literal question.)
Your Linux platform is long out of service. Also, honestly, if you know how to download the source code of transmission and try building it, you're already more proficient with Linux system administration that most of Ubuntu users are. So not sure how much you benefit from Linux lite's restrictions:)
Maybe going for a mainstream Linux distro like xubuntu, which is just like Linux lite, xfce running on the Ubuntu distribution base, just with a much richer selection of software and a much larger user base to draw from, makes your life easier. In your specific case, not going Linux lite but an official Ubuntu spin like xubuntu would have allowed you to directly install transmission via apt.