According to the skill in the android app, it says

This skill requires account linking,  If you have multiple profiles in your household, Alex will only enable and link this skill for the profile that is currently logged in.

IMO,it is saying you will not be able to do what you want.

Both Dot 3rd gens running latest firmware?

I have a dot 2 and a dot 5, sound quality is clear with no problems.

However, the 2x dot 3's I have, the sound from Alexa's responses is completely unintelligible at the worse and bad at the best. No top end, just muffled like it is covered with a pillow.

The EQ settings in the app make no difference whatsoever to Alexa's spoken responses, no change - EQ does not seem to touch Alexa's responses.

EQ does however change the quality from any streaming of Amazon music or streaming radio stations and it is quite acceptable.

I talked to customers services who were assuring me this seems strange and they haven't heard of it before 🤔 I sent one back for an exchange..which is exactly the same.
Warranty period expired just after that so I'm stuck with it.

Anyone else or just me? If so, is there a fix for it without using external speakers?

Thanks

Anybody have any advice on how to do this?

Sorry to say it, but it looks like Amazon is making things worse.  I just visited their Alexa site and there is a banner that reads:

The site address I visited was alexa.amazon.com

"This website does not currently support all Amazon devices and Alexa features, and functionality will continue to be reduced over time. For full functionality, please download the latest version of the Alexa app. Contact us if you have any questions."