How would you classify the Four Humors on the Three Suitors
Oh well Jack is clearly Melancholic, which is frequently code for Clinically Depressed, and translates well to Sopping Wet Poor Little Meow Meow Disaster Man
Arthur you can probably make a Sanguine reading stick to. He's cheerful, sporty, and gregarious (and so full of Blood!) And then he spends most of the novel crying (oops)
That kind of leaves Phlegmatic for Quincey but...eh? He is both laconic and emotionally understated, which fits, and Jack describes him as the cool-headed leader, which also fits, but on the other hand he's a storyteller, he's decisive/action oriented (is he impulsive? Maybe? He shot a gun into a room full of his friends fer crying out loud) and his whole thing with doing the bit for Lucy and then arranging boy's night is a very different feel from the Phlegmatic archetype. So I think I want to say, if you have to put him somewhere that's the place to put him, but he's not (or not solely) built on that archetype, at least in part because he's also built on a lot of American archetypes.
And because I think Stoker is not trying to build out the boys as a Four Humors squad. I think Head, Heart, and Hand is a better framework for them.
That said, I think you can slot Jonathan fairly easily into Choleric to round them out. On the one hand he's very, restrained, quiet, and submissive. Mina says "quiet dignity," Jack says "unassuming and professional" - our boy is understated. So it's tempting to read him as Phlegmatic, and he can certainly do the "cool and logical" thing. But we know that he does have a lot of big emotions lurking under there, and most of them are Rage. Our man is incandescently angry. And not without reason! (So why isn't this like Arthur crying? ...idek I just work here)
Maybe a better way to think about it is that individually each of them is much more complex than a humorous archetype, but within the dynamic of the Drac Attack Pack they inhabit a Four Humor's Squad much more naturally. In that context Quincey behaves as Phlegmatic and Jonathan as Choleric, even though their characters as a whole do not necessarily inhabit those archetypes.
That said, van Helsing is a pretty solid example of a Sanguine archetype - he eats and drinks and laughs, is overflowing with confidence and presence, and does much better with a friend or an audience or both. He's overstated and enthusiastic. As much as he tries to paint himself as a Sad Old Man, he's got plenty of bloom and blood to him.
And Dracula is pretty clearly Choleric. Hey, there's another Dracula-Jonathan parallel for everyone!
Mina I think inhabits the Phlegmatic archetype better than anyone in the cast - she actually is cool-headed and logical, and when she's not she notices that as anomalous and is cool-headed and logical about it. She's not emotionless or dispassionate by any means, but she is not ruled by her passions, and she's cool under pressure.
Renfield is explicitly described as Sanguine (by Seward), but like everyone else, Renfield has a Lot going on, and I am wondering if he would have changed his initial assessment by the end of that story