navigate to C:\Hitech Creations\Aces High III\films or where ever you installed the game. Right click on a film and select "open with". In that drop down, if it doesn't have the AHfilm, use the "browse to...." to browse to the Aces High III folder and click on the AHfilm.exe.
This will set the AHfilm viewer as your default program to open to run the films.
As far as I know, AH2 films cant be viewed by AH3 film viewer.