I have looked for the adobedtm script/url on the Three login page that gives rise to the Webshield popup.
hxxxx://assets.adobedtm.com/acccca982240/2889efaf6b3f/launch-e9ff484f5018.min.js
I submitted that url to Virustotal and shows all as Clear. But Avast/AVG not listed.
I have had no reply from Avast re my FP form

I have also reported the login issue to Three.