Sniffing gigabit ethernet? 1000baseT LAN taps?

[Michael ODonnell]

I don't know what your situation is but if there's a managed
switch involved I believe that some of them can be rigged to
echo traffic to one or more specified ports for analysis/debug.

Or if your problem isn't specific to GigE (and you can tolerate
10baseT speeds) I have a little NetGear hub you might be able
to hook all three lines into for eavesdropping...