Try running the editiors via emulator. in Windows you can right click on the icon for the program, select properties, and then the compatibility tab to emulate to a previous system
Third mission should be straight forward, read the hints and you will find out which Imperial craft target the B-wings and Maria and go after them. The Maria can hold her own against the Frigate, it is the bombers that do the most damage
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Official XWA flight instructor
get busy with it!