my old iPad 3 will not update

No.

This is simply NOT possible.

If you are worried about security, I would STOP using that 2-2-1/2 year old Safari web browser, on your iPad 3, 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 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

Good Luck to You!

Ipad 3rd gens max out at iOS 9.3.5 I believe and cannot be updated to iOS11

Old iPads, like the iPad 2, 3 and 1st gen iPad Mini cannot ugrade their iOS past iOS 9.3.5.

Sorry.

There seems to be no security updates for older iPads. Would linking to a computer do it?