Can i hear my phone conversations on ultra 3 watch in my hearing aids?

Many high end hearing aids offer an iPhone app that can allow you to control the output via your iPhone, and send the audio of an iPhone call to your hearing aids via bluetooth. You need the App for the brand of hearing aids, and your hearing Aids must support that feature.

I use that and it works really well.

I don't know whether you can then generalize that to include your watch as an I/O extension of your phone.

I can't really imagine wanting to do that when you can play the iPhone directly into your hearing aids.

...or maybe I am not understanding what you want to accomplish?

Are you using a cellular-connected Apple Watch as your only device?

My Resound app runs on my [GPS-only] Apple Watch (set up as a peripheral of my iPhone).

It can control the hearing aids, but I honestly have not tried whether that could be configured stand-alone to send call audio to the hearing aids when an iPhone is not present..