How to update my I pad 3 to ios12

The iPad 3 ( 3rd generation ) is a six year old device, now.

Its hardware is too old and underpowered to handle any other iOS version than its current version of iOS 9.3.5.

In addition older iPads, like the 2010, original, 1st gen iPad, iPad 2, 3, 4 and 1st gen iPad Mini are all 32-bit hardware and software devices.

iOS 11/12 and later iOS versions are all 64-bit coded operating systems and coded for newer iPads that have 64-bit hardware, now.

Older iPads and some older apps are incompatible with iOS 11/12, now.

So, all around, this will never be possible.

Tap and read my full explanation here.

https://discussions.apple.com/docs/DOC-12481

In addition, I would STOP using that 2-2-1/2 year old Safari web browser, as it is, probably, not safe and secure to use any longer, and seek out a third party web browser that is STILL currently being updated and STILL being supported on iOS 9.3.5.

Here are a few other third party web browsers that I know are STILL being updated and supported on iOS 9.

Perfect Web browser

iCab Mobile Web browser

Dolphin Web browser

Aloha Web browser

Maxthon Web browser

Opera Web Browser

Perfect Web Browser JUST had a very recent update to iOS 11, but the older last version update is still available here and had its last older iOS update this past April 2018.

Perfect® Browser+ (Legacy) by AppSimply

https://itunes.apple.com/us/app/perfect-browser-legacy/id373916467?mt=8

Good Luck to You!

Sorry but the iPad 3 cannot be updated beyond iOS 9.3.5.

See the following for a list of devices compatible with iOS 12: iOS 12 is compatible with these devices - Apple Support