Can the 30W charger be used with the iPad Air 3 for fast charing?

Fully understood. That is the very Power Adapter that I use with my various iPads - noting that the 30W Power Adapter (and the prior 29W version) predate the much more recent and physically smaller 18W Power Adapters.

I cite the Mophie PowerBank as an example of the usefulness of the higher-power Power Adapter - as an answer to the final question in your original post. There is benefit in choosing the 30W Adapter, over the 18W option, but not to the iPad.

iPad Air3 supports USB-PD (Power Delivery) Fast Charging - at a maximum rate of 18W (i.e., 12V @1.5A). Using a higher wattage Power Adapter, such as 30W or greater, will not shorten iPad charging times - but will do no harm.

A higher wattage Power Adapter does have its uses. For example, in addition to potentially powering a Mac, some PowerBank batteries can also be Fast Charged - and can benefit from a higher wattage Power Adapter. A perfect example would be the Mophie Power Station 3XL:

https://store.apple.com/uk/xc/product/HM8X2ZM/B

If purchasing a third-party Power Adapter, you should be aware that their are two Fast Charging standards - these being USB-PD (Power Delivery) and QC (Quick Charge). Apple does not support QC.

Fast charge your iPhone and iPad – Apple Support

I hope this information provides the definitive guidance that you’re looking for.

Your iPad Air 3 shipped with a 12w power adapter (maximum 2.4A of current). Your iPad can actually accept as much as 2.2A at a time. You can try to supply more than 2.2A, but that's all that it will consume. It is not dangerous to connect a power supply that can provide 6A (30w) of electricity, but your iPad will only drink 2.2A worth.

In other words, I may be able to cook 30 hamburgers an hour, every hour, but you can only eat 2 in the first hour and none for hours after that (until you get hungry again). No matter how fast I cook, you won't eat faster than your body allows.

You can use the 30w charger safely, but your iPad will not charge any faster.

The second link repeats information that I have provided here.

The iPad Air3 can use a 30W (or higher rated) USB-PD Power Adapter - but will only Fast Charge at the 18W rate.