How to upgrade IPad 3G to IPadOS 18?

You cannot. Apple introduced the iPad (3rd generation) roughly 13 years ago. For that iPad, which has a 32-bit processor, iOS 9.* is the end of the line. You cannot update it to iOS 10, let alone to iPadOS 18.

An iPad 2, iPad 3, or iPad mini 1 isn't capable of 10 or above; they require an A6 or better CPU.

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