Charge iPad Air 3 w/ 61W Macbook Pro adapter?

Your iPad Air3 supports PD (Power Delivery) protocol Fast Charging. If connected to a PD-compliant charger, such as your Mac power adapter, the iPad will charge at the highest rate supported by your iPad.

Your iPad will achieve its fastest charge rate when connected to an 18W (or higher) charger - although connection to a charger greater then 18W will have no benefit or detriment.

As such, your Mac power adapter will provide the maximum 18W charge-rate to the iPad; this may be greater than the standard adapter that may be supplied in the retail box of the iPad - this typically being a 12W brick.

I hope that you find this explanation to be helpful.

It will charge the same. The Lightning port specification calls for 9W max (if I remember correctly), so if you plug it into a higher charger, it will simply draw less than the charger's full capability, but there's no issue with doing it.