does ipad air 3rd generation support 5G

No - iPad Air3 does not support 5G Cellular networks.

Support for 5G Cellular networks should not be confused with 5GHz WiFi networks.

Only the 2021 iPad Pro models support 5G Cellular networks - and of those only those models sold in the United Status support 5G mmWave networks.

No.

That iPad year and model was designed and made before 5G support was adopted by Apple.

Sorry.