We carried 3 dogs in harneses on leashes yesterday, in addition to a mom and her 8 4-wk old pups.
Mom + 8 traveled well in a large wire collapsible kennel, 36"x27". We pulled a blanket most of the way up around the sides of the kennel, since mom was very protective of her pups and some growling went on until things settled down.
The leashed 3 were 2 female golden retrievers and a male boxer. All had been neutered, and the foster family who had the dogs together for only one night reported that they got along well, no problems. We confirmed this the next morning when we met them at the airport. All the dogs had come from different shelters, i.e., they hadn't previously been together. They traveled well in close quarters for an hour and a half. The older golden got tangled in her leash and did chew it off... No problem she was content to lie down and snooze. And Jill was able to get back to her and re-fasten the restraint. Good reason to not fly solo under these circumstances.
But... that's the second leash we've had chewed off, so we're going to chains for tethering. My wife and I have discussed "emergency" procedures and agree that any hostility between dogs prior to a tethered flight is reason to leave one behind or cancel altogether.
We have always covered our interior with blankets and towels, and recently began to place a plastic tarp under everything. Anything liquid will hopefully get soaked up by the old blankets, anything that get through will be stopped by the tarp. We've had to clean up all of the 3 "P"'s (not on the same transport), and as Jon says, hosing things off is easier than cleaning out the airplane
Most flights have been very routine, but we've got to be ready for anything, anytime.
We turned our TC-310Q into a freight dog for this one, and, of course, did a revised wt and balance for no back seats. The leashes were secured to cargo rings on the floor and the kennel was secured with a cargo strap. As Debi and Jon have empasized, safety first.