I have RSA pubkey auth set up on a few machines, so scp works without
asking for a passphrase for those machines, when one file is remote
and one file is local:

  scp justinpryzby.com:s.php /tmp
  scp /tmp/s.php justinpryzby.com:

It also works when both files are remote and on the same host:

  scp justinpryzby.com:s.php justinpryzby.com:t.php

But it doesn't work when transferring between distinct remote hosts:

  scp justinpryzby.com:s.php cyberia.stlawu.edu:/tmp

scp -v indicates that authentication is successful for j/p.com, but
fails for cyberia.  It seems that scp is connecting to j/p.com, and
then sending an scp command requesting that s.php be sent from the
localhost (j/p.com) to cyberia.

I agree that it is not optimal for files to all be transferred to my
local host (the one sending the initial scp command) as an
intermediate step, and that it is more efficient to connect to the
ultimate destination machine.  Then, only some commands must be sent
between me and it, and not an arbitrarily large amount of data.

But it isn't intuitive that this is how it works.

If this is intended, then I think it should be better documented:

found 351489 1:5.9p1-1
thanks

I have the same kind of problem (without the -3 option).
connects to host2 (BTW, that's why it should be documented, because
both ways are not equivalent). It should also say how the hosts are
interpreted: both hosts related to the local .ssh/config, or host2
related to the .ssh/config on host1?