Maximizing old iPads
Can I use my old iPads (iPad 2 and 3rd Gen) as second monitors for my MacBook Air- Sierra OS and iMac - High Sierra OS?
Thank you!
MacBook Air 11″, macOS 10.12
No. Neither your iPad2/iPad3, nor Mac with High Sierra, support Apple’s Sidecar feature:
Use your iPad as a second display for your Mac with Sidecar – Apple Support
Similarly, third-party alternatives, such as Duet Display which do support High Sierra, require an iPad with iOS10.x or later. iPad2 and iPad3 are obsolete - and cannot be updated beyond iOS 9.3.5 (WiFi Only) or iOS 9.3.6 (WiFi+Cellular).
https://apps.apple.com/gb/app/duet-display/id935754064