sonos amp with yamaha sub and a pair of play 3's

No. Or at least, I don’t think so. The Amp requires, I believe, a pair of speakers before the line level subwoofer output is activated. The PLAY:3s would be set up as a second ‘room’ in the Sonos software, and have no impact on the subwoofer in any way. 

I thought as much though some of the online topics seemed confusing. I did try it with no luck. Looks like the only option is the Sonos wireless sub.

Or replace the PLAY:3s with a passive set of speakers connected to the Amp, which should then send a line level output through the sub output port. That’s assuming the Yamaha accepts a line level input. 

Yeah and setup an isolated analogue type arrangement just for my record player and amp. I do find the whole line input integration very unintuitive.

Would the sub work if I connected the play 3s to the amp using ethernet cable?

It’s not an ideal setup because the PLAY:3’s would not have the benefit of the AMP’s subwoofer crossover, but you could connect the turntable or CD player to AMP, then Group the PLAY:3’s with AMP. There is a second, very subtle, potential issue that some listeners will experience, most will not.

All members of a Group are time aligned. This time alignment has always been a unique, patented SONOS feature. As music is sent out over the network to a bunch of speakers, there will be momentary stalls in the network traffic potentially causing some speakers to fall behind. You may be aware of the 75ms latency between SONOS Line-In and output from the players. This latency gives each player some time to adjust for varying network delays. If you are very sensitive and paying close attention, it will seem like the musical “image” will shift slightly while an adjustment is underway, Like I said, some listeners will notice this adjustment, others will not. If you don’t notice, don’t feel like you are in a disadvantaged bin, in my opinion you are in the fortunate bin. Unfortunately, I’m in the unfortunate bin. This is an occasional, well done adjustment that does not gross me out. In real terms the sonic consequence of the adjustment is similar to you slowly moving a speaker location a couple feet.

No, I wouldn’t think so. There isn’t really any setup that would ‘work’, beyond what I suggested above. There just aren’t any Sonos devices designed just to work with a subwoofer. 

Or just buy another Sonos sub lol

Heh. Yea, I should have said to ‘push’ a third party Sub at the same time as using Sonos speakers. 

Fyi I just tried Buzz's suggestion of grouping but still nothing. It seems as there are no speakers physically wired and connected to the amp it disables the sub and just plays through the play 3’s. So either a Sonos wireless sub or another pair of wired speakers.