First off, I am so sorry.
I know you are feeling like crap now but I will tell you don't!
You are a GREAT mommy to Ringo and Lucy! In the end, dogs will be dogs. We don't always know why they do what they do.
Jackson has been in two big fights, both with his two closest dog friends (my dads dogs) who he lives with part of the time. They went AT it, but it's only happened so far once with each dog. The second fight with him and Buddy ... they both had a little scab on them, Jackson on his ear, and I think Buddy had one on his belly. I know it was from the fight but... I didn't worry too much about it.
Living with multiple dogs, fights ARE most likely going to happen. I think you are lucky this is the first! Sometimes they never do, but there is always a chance. Don't feel too guilty -- Ringo will get over it (the fact that you took Lucy alone) and I am sure Lucy will forgive him quickly. Dogs have a way with that. Buddy and Jackson were practically trying to kill each other (over a bully stick) and 5 minutes later, were laying next to each other again.
All 3 dogs are always left alone with each other. We don't live bones out or anything they could think is "protective worthy" but they just sleep while we are gone anyways.