Your iPad Air 3 supports USB-PD Fast Charging at 18W. A higher-wattage Power Adapter will do no harm as the iPad will regulate its maximum charging rate as needed.
As such, you will achieve the maximum iPad charging rate using an Apple 18W Power Adapter (charger). However, if purchasing a new Power Adapter, a 30W Power Adapter may be useful if you have any devices that would benefit from a higher wattage. For example, you might use an external Power Bank - that itself may be recharged from a USB-PD compliant charger.
I hope this clarification and information is helpful to you.
It is worth noting that your iPad will not necessarily charge continuously as its maximum rate. The charge-rate is always reduced as the iPad exceeds 80% charge, or if the CPU is under high load, or in high ambient temperatures.