How can I export MIUI notes?
Choose an option:

Simple copy-paste.
Share/send note option - whichever exists - thereby sharing (exporting) it to the app (and hence format) you prefer.
Screenshot, because that's a readable format too, logically speaking.

Don't know what the version of notes is in your Redmi 3s. Probably very outdated. Otherwise in newer apps, you have a not so good export feature.