FP3 - storage too full and phone behaving strangely- how can I scan for virus?

It’s not straightforward because phone OSes (not only Android) are doing their best to restrict what Apps can actually do in general. That is true for potentially malicious Apps as well as for supposed virus scanners.

https://web.archive.org/web/20200806231715/https://www.androidcentral.com/antivirus-android-do-you-need-it

Did you try safe mode already? … [HOWTO] Reboot into Safe Mode