Thanks. Your answer made me go out and take a closer look. You were right, there was a fourth hole - but it wasn't on the bottom of the tank the way the tank was positioned.
In my Biggar
, the freshwater tank was laying on its side under a couch. The fourth hole was higher than the hole leading to the pump but lower than the hole for inserting water and what I guess is the hole to the air vent. So the tank had to be taken out and tipped upside down to get the rest of the water out. The tubes were not glued on. They connected to plugs that screwed into the tank. Unscrewing them worked worked to free up the tank. More elbow grease to pull the tubes off of the plugs might also have worked. The fourth hole looks like it originally was about two inches in diameter and screwed, but at some point had been melted or glued shut and the replacement seems to be a tiny hole with a metal screw with a rubber washer.
Once the tank was off, I could stick one end of a tube into the tube that led to the pump and the other end into the non-toxic antifreeze container, then let the pump move the anti-freeze through the system.
Don't know whether all biggars were made as mine is or whether my set up is some modification by a prior owner.
Thanks again for your help!