That's only four cars. I think we're told when we have too many, and until then, its best not to even think in those terms. My wife once told me I had too many boats in the house, garage, and barn, and that's how I knew. I didn't think it was possible to "have too many boats". So I sold about 6 kayaks and now I don't hear any more about that.
I've also got four, with one being driven by my youngest, who will leave for college 500 miles away in August, so we'll be back to four for two drivers, plus another for my oldest who just bought a new Honda Element and will be coming home to manage a business after college. So we'll have five here. I'm out of room for building more barns and garages.
The three cars that you mention are all highly desireable vehicles. I suspect I'd try to keep them all - until someone told me there were "too many". Then I'd have to ask her which one was too many after protecting my top draft choice - the pickup (unless you are close to me in MD, in which case you should sell me the pickup, as I only have one). In seriousness, the Mustang probably has the most value and would be an easy sale at a time of year when convertibles bring the highest price (now). But its also your only convertible, so a case could easily be made to keep it. This is a very tough decision and should probably be discussed over beers. Good luck!