Your dad's 'bike is definitely a Douglas Carl. The horizontal twin cylinders give it away. Weren't many of that type of engine about apart from BMWs, and it certainly isn't one of those. Not sure about the other two 'bikes though.
Pleased you found the photo's. I'm ploughing my way through hundreds of photo's, slides and negatives and getting them onto the computer. Very rewarding but can also bring a lump to the throat at times.
Cherish them.
Ex classic 'bikes owner,
Chad.
PS. If you send a copy of the photo's to the Vintage Motor Cycle Club, someone will give you a load of information about the 'bike and probably the other two as well.
www.vmcc.net