Jen, you should buy whatever primes whose focal lengths you need.
Don't buy a 'holy trinity' just because of its name. The FF trinity is usually either the 35/85/135 or 24/50/135, but neither of the crop-equivalent sets may actually work for you. You say you're happy with the 30/1.4 and 85/1.8, which is fantastic. Will you enjoy the 135? It's hard for us to say for you, because we have no idea whether you like the 135mm FL on crop.
As you said yourself, you've got the 50-135, so shoot at 135 for awhile and ask yourself whether you like that FL. Don't just buy a 135 because it's an L or that it'll complete some sort of silly trinity.