The Send All New Firmware to P3 button is enabled only when there is new firmware available to send to your P3. All the required firmware files necessary to load your P3 must be present for the button to be enabled.
The Send All New Firmware to P3 and Send Checked Items to P3 buttons are disabled while copying files from Elecraft.
If the button is not enabled when newer versions are shown in the Available column, try clicking Copy new files from Elecraft to copy the required files from the Elecraft server.
If the Send All New Firmware to P3 button is still disabled, required files may be missing from your Local folder for firmware files. See Missing Files.