While both the AIR-PWRINJ and AIR-PWRINJ3 are listed as Aironet power injectors, they don't support the same Aironet radios.

These power injectors provide up to 15 watts over a Category 5 Ethernet cable, supplying enough power to support up to a 100-meter cable run.

The AIR-PWRINJ is compatible with the Cisco Aironet 350 Series Access Point and bridges. The AIR-PWRINJ3 is compatible with the Cisco Aironet 1100 and 1200 Series Access Points.

Cisco Aironet 350 Series Access Points and Bridges
Supported Power Injector: AIR-PWRINJ
External Power Supply: None Required

Cisco Aironet 1100/1200 Series Access Point
Supported Power Injector: AIR-PWRINJ3
External Power Supply: AIR-PWR-A (required)

Note: Cisco AIR-PWRINJ3 power injectors can be used with Cisco Aironet 350 Series devices, but the AIR-PWRINJ power injectors cannot be used with the Cisco Aironet 1100/1200 series devices.