In this case, it's not the #!-line you should be changing, but your $PATH.
#!/usr/bin/env python3 This has the effect that the python3 interpreter will be searched for in the directories listed in your $PATH variable.
If python3 is installed in, e.g., $HOME/local/bin, then this directory must be present in $PATH before any other directory that may also contain a python3 interpreter, otherwise that one will be used instead of your own in $HOME/local/bin.
So you need to set
PATH="$HOME/local/bin:$PATH" Either on the command line, or in a shell initialization script (~/.bashrc for example). C-shell users do it differently...
set path = ($HOME/local/bin $path) In general, the #!-line identifies the interpreter for the script, i.e. what program to use to parse and run the file. A sed script, for example, may use #!/usr/bin/sed -f.