man scp says:

-3 Copies between two remote hosts are transferred through the local host.
Without this option the data is copied directly between the two remote
hosts.  Note that, when using the legacy SCP protocol (via the -O flag),
this option selects batch mode for the second host as scp cannot ask for
passwords or passphrases for both hosts.  This mode is the default.

I understand "This mode is the default" as that your copies will be through
your host (meaning I don’t need to pass -3 - it is default), but if so, how
do I get different behavior, where files being copied directly between two
remote hosts? Man page says "without this option", but how can I not pass
it if it is “default”?

Not native speaker.