You will not be able to update past iOS 9.x for the 3rd gen iPad.
https://everyi.com/by-capability/maximum-supported-ios-version-for-ipod-iphone-ipad.html
The apps you will be able to download will diminish within the next year or so as software developers switch to the new versions of XCode which will exclude support for iOS 9.
Furthermore, Apple has confessed to slowing down older devices and hiding that fact.
http://money.cnn.com/2017/12/21/technology/apple-slows-down-old-iphones/index.html