At minimum, setting the variable Python3_ROOT will ensure the search will be limited to this root directory.
Now, to have a full control of the artifacts selected, without control of the find_package() command is to define, at minima, the following hints:
Python3_EXECUTABLE
Python3_LIBRARY
Python3_INCLUDE_DIR
One possibility, at the top level, is to execute a CMakeLists.txt with the correct find_package() command and reuse the results of this search to set the hints.