Can anyone tell me of a single calendar app that will sync with my Outlook 2010 calendar on my Windows 10 PC, my microsoft Lumia 640 windows phone and my Android tablet?
Why not use the built in calendar on your Lumia?
You can just use the Outlook app for calendar only on Android and it should sync just fine. Remember to give Gmail all permissions on your tablet. I'm not sure how it works with PC but provided that you use the same accounts, they should sync.