Why my iPad 3rd generation not able to update to iOS 13?

iOS 13 is not available for iPads. And if you're thinking of iPadOS, well it's strictly 64-bit. And it won't upgrade past iOS 9 anyways. It's not even a matter of performance but that anything newer would require a 64-bit processor.

Never mind.

YOU mis-typed in your post title.

You have an iPad Pro 3rd gen. NOT a 7 year old iPad 3rd generation.

iPadOS for iPads is scheduled for release on Tuesday, September 24th.

iPadOS 13 has not been released yet. It should be released on September 23.