Ideally, you'd have a composite slalom boat, a composite wildwater boat, and (these days) a composite freestyle boat.
In reality, most people will have all plastic.
Ok to race your plastic freestyle boat, plastic slalom boat, and plastic freestyle boat.
Since there is so much focus on the overall scoring, if you choose to be in all three events, it's ok to break out the old school boat for the wildwater section, and perhaps even the slalom section.
You can even use your creek boat for these.
I'm sure that you can get a few spins and an ender in an old long boat or your creek boat.
So, don't worry about having the perfect boat. Just come on out and participate. You never know.
After taking part in the Triple Crown, you might just look back on it as the discovery of some new aspect of whitewater that you never before realized was fun.
(boats in the top photo, from left to right: SL350, Reaction, Wavehopper, MadBoy, Fish)