Heat it with a torch, then try two pipe wrenches. If it doesn't bust loose, go to the next joint and turn it loose there. If that doesn't work, cut the pipe and use a flexible coupling with a metal band around it to make the connection.
Your best option is to start on one end and take the galvanized pipe out, especially if it is old pipe. Mineral deposits from the water and other gunk build up in galvanized pipe over time, causing the problems you have now and eventually clogging things up.
Use PEX, copper or CPVC