Strange failure, are they DOA or do the phone fail after a few weeks? I don't see how you could have three phones with a defective microphone, it's a pretty simple piece of the phone. I wonder if you are holding the phone in an unusual way or if you are a low talker or if you are doing something that makes it think there is noise and some noise suppression code is causing the mic to cut out.
As for the general question about replacing the GN with an S3, I'd wait to see the S3 before I made the decision. Some reviews of the S3 have said that the screen is less bright than the GN, that sounds unlikely to me but if true it would be a reason to stick with the GN. Otherwise the S3 has a few improvements over the GN, none of them very important. The CPU is slightly faster, but I doubt anyone would notice that because the GN is fast enough. It has a better camera, that's nice but not really important. It has 2G of RAM, that's a feature that's potentially very important for future updates. It has a uSD card slot, that's nice but with 32G of FLASH in the GN I don't think that actually matters. It also has HDMI out, sounds cool but probably useless. The slightly bigger screen might be nice, but not if it's dim. The downside is that it's not a Nexus device, however the Verizon GN isn't being treated like a Nexus device anyway, who knows when we'll see Jelly Bean. It's possible that the S3 will get Jelly Bean before the GN.
If it were me I'd take the S3 if I liked the screen otherwise I'd stick with the GN.