When I was thinking about getting another cat, I read that it was a bad idea to have odd numbers of cats, since they pair up and one ends up as the odd cat out. I fully expected this to happen with Rugrat and Lucy, but didn't think that Luna would mind being the odd cat out, since it would mean that Rugrat would pester her less.
However, this hasn't really happened. The cats spend most of their time divided into a group of 2 and a loner cat, but the composition of the groups is really fluxional. In general it's about equally divided between Lucy and Luna or Lucy and Rugrat, with a smaller portion of time spent as a group of three or with Lucy off on her own.