If you want to find the default Python 3 version of any Ubuntu release, you can run the command
rmadison python3
python3 | 3.2.3-0ubuntu1 | precise | all
python3 | 3.2.3-0ubuntu1.2 | precise-updates | amd64, armel, armhf, i386, powerpc
python3 | 3.4.0-0ubuntu2 | trusty | amd64, arm64, armhf, i386, powerpc, ppc64el
python3 | 3.5.1-3 | xenial | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
python3 | 3.6.5-3 | bionic | amd64, arm64, armhf, i386, ppc64el, s390x
python3 | 3.6.7-1~18.04 | bionic-updates | amd64, arm64, armhf, i386, ppc64el, s390x
python3 | 3.8.2-0ubuntu2 | focal | amd64, arm64, armhf, i386, ppc64el, riscv64, s390x
python3 | 3.8.6-0ubuntu1 | groovy | amd64, arm64, armhf, i386, ppc64el, riscv64, s390x
Alternatively, check the package
https://packages.ubuntu.com/search?suite=groovy&keywords=python3
For groovy (Ubuntu 20.10), it's Python 3.8.6:
