Consider the Sonos Port like any other device connected to your amplifier. It has one set of RCA out jacks that you cable to a line in on your own amplifier. 

There’s not enough information in your post to tell if a Port would replace a receiver, there are too many other questions. Is this receiver powering to speakers? What kind (impedance)? If so, you’d need a Sonos Amp, most likely. Both the Port and the Amp have a single line in, so if you use your receiver for multiple inputs (as a switch), you’d likely need to get some sort of switch to access those multiple inputs. 
 

So the receiver powers one set of speakers. The rest are powered by two external amps. I have 4 sets of speakers connected to the external amps (2 to each). The receiver (Yamaha RX-V3000) is set at 8 ohms (can be set to 4). I think I have power to the speaker so I guess the port should do the trick connected as an input to the receiver. Correct?

Below are the specs for the 2 identical external amps.

I think so. You can think of the Sonos Port as any other input device, a tape deck, a CD player, etc. It has an output that is line level, just like all of those devices. It is merely another source of a signal, attached to an input on your Yamaha.