No.  You cannot connect Era 300 speakers directly to the TV.  You need an Arc or Beam 2 for the front, and the Era 300s connect wirelessly as surrounds.

Hi

The Era 300 series speakers were never intended to be used as a TV speaker the same as a Sonos Arc or Beam2.

The Era 300 series are intended to be used as a stand-alone speaker...single or stereo pair. They can also be used as surrounds with the Arc or Beam2.

They can also be used to provide the audio for a turntable with a built-in pre-amp or 3rd party pre-amp that sits between the two using the Era 300 line-in.

Your profile shows you own 1 or 2 Era 300’s ... hope you can still use the speaker(s).

Let me further clarify….you can Group the Era 300 to the Beam for streaming music either via a streaming service or music sent from your device.

What you don’t want to do (if the Era and Beam are in the same room) is to Group them for TV/Movie audio; as there will be a 75ms delay to the Era 300. That delay causes an echo effect.

If the Era 300 is in a different room Grouping for TV/Movie audio may be permissible where the delay would not be heard. Think listing to a sporting event commentator in another room without the video. 

Sorry to hear that the ERA 100s do not work with a TV.  As much as my wife loves wires and speakers spread all over the house I was hoping to utilize the ERA 100 to get sound from my television. I did purchase the three adapters, the one for the Internet cable, the other for the HDMI and the third one which converts the optical into HDMI.  Sadly, none of these adapters do anything to support television sound.  It would be nice that when selling those adapter dongle that they actually explain how they function, and where they will, and will not work.   This would save time money and frustration.  I did attempt to search for ways you can use the adapters to make the TV speakers work, but after reading the other conversation, I realize it’s not designed to work that way.  Hopefully you can explain to my wife why we need so many different sets of speakers to accomplish the task.  
frustrated by the lack of clarity.  
if a workaround becomes available, please let me know.  For music the Sonos system has been great no issues in that department.  

I can see you are disappointed because the things you bought are not working like you thought they would. Sorry to say I do not agree with you this being about a “lack of clarity” on Sonos’ part though. Buying expensive equipment not only takes ticking all the boxes in the shop and then presuming it will work, but (if you do not want to be disappointed) also needs a bit of effort by the customer himself.

From: https://www.sonos.com/en-us/shop/sonos-line-in-adapter

“Connect an audio source to Era 100, Era 300, or Move 2 using this 3.5 mm to USB-C adapter. Auxiliary cable not included.”

This does not mention HDMI, nor optical. So how did you get the idea it was possible to connect a TV? At least you could have read up on connecting devices to your TV.

As a work around you could try to connect the Era’s via bluetooth to your TV. Not sure this would work though.

Or maybe a headphone jack?