Tough one, The DC trinity is basically about who are the three that is the standard by which all others compare. 
 
Personally if I had to choose the 3 heroes every marvel character should be compared  I would say: Captain America (Steve Rogers specifically), Spider-man, the Mighty Thor. 
 
Say what you want about your personal tastes but these 3 are real heroes, The guys that the marvel universe can look up to, who uphold the highest ideals, inspire people and never give in to despair and corruption no matter what's thrown their way.

Well.  
I think officially it's probably considered to be: Captain America, Thor, and Iron Man.  
That's the Avengers Trinity at least.    
  
Their most popular characters would probably be like... Cap, Spidey, and Wolverine.  
My favorites are Spidey, Silver Surfer, and Hank Pym... But, that's definitely not the Marvel Trinity.   Hahaha    
 
Their most important ones are probably really Captain America, FF, and Spider-Man though. Chronologically. It's harder to pin down a trinity for Marvel. But when you say "Trinity" in a Marvel context I automatically think of Cap, Thor, and Iron Man. 
 
Sorry if that was rambly, hah. 

Well the DC Trinity exists on two levels.  Both the importance to the characters, and because in the real world, they are the 3 best selling characters.  Almost every DC hero looks up to or is affected by one member of the DC Trinity, or falls lightly upon one of their shadows.   
 
So in making a Marvel Trinity, you have to consider both the shadows cast to other characters and their sales. 
 
So I might recommend Spider-Man, obviously, for being Marvel's flagship character.  (Marvel admits this, more or less, in the Visual Guide.)  The X-Men are also a huge influence and sales driver, so I'd recommend Professor X too.  HOWEVER, Wonder Woman adds a female perspective to the trinity, so instead of Professor X, maybe Jean Grey or Emma Frost could represent both mutants and women in the Marvel Trinity.  And lastly, Captain America because EVERYONE looks up to him, even Spider-Man.  Even some villains look up to Cap. 
 
Spider-Man 
Emma Frost 
Captain America 
 
But...  They'd never get along very well.  And the DC Trinity in and of itself is almost a team.  Hell, even their sidekicks (Robin, Superboy, and Wonder Girl) get along and were a major force in the Teen Titans.  Marvel has always been more "realistically disorganized" then DC. 
 
Know what?  Marvel may not even have a "Trinity".  Just throw in the Illuminati team to serve the same function.  Although, Spider-Man's not in the Illuminati and neither were any women.

Spider-Man and Captain America are givens imo. I think either Iron Man or Thor would be the third.

Well, that is the thing about Marvel. 
 
It has its big three in money makers currently: Spidey, Cap, and Wolverine 
Holy Avengers Trinity: Thor, IM, Cap 
Holy X-Men Trinity: Wolverine, Jean, Cyke

Thor Cap and Iron Man no doubt Marvel is sexist

spiderman thor an captain america

" Well, that is the thing about Marvel.  It has its big three in money makers currently: Spidey, Cap, and Wolverine Holy Avengers Trinity: Thor, IM, Cap Holy X-Men Trinity: Wolverine, Jean, Cyke "

I don't think Marvel pushes the trinity thing in the same way DC does.  That said, the most iconic characters in Marvel are:
Captain America
Spider Man
Fantastic Four 
Hulk
Thor
X-Men   

Spider man is the most iconic character in Marvel then goes Wolverin and Hulk.

" Marvel has more iconic teams than characters.  The Fantastic Four, Avengers, and X-men being the biggest.  Also Marvel kind of set the standard for antiheroes, so characters like Punisher, Hulk, and Wolverine could be considered iconic from that standpoint.  As far as a big three... I don't think there really is one.  Spiderman, Captain America, and Thor would be my personal picks.  IronMan, Daredevil, Wolverine, and Storm could be just as easily be inserted in anyone's place  though. "

I don't think Marvel has a set 3, but they do have a handful of characters they tend to highlight.
Which I think are:
 
Spiderman 
Hulk
Iron Man
Ms Marvel

"Spider-Man and Captain America are givens imo. I think either Iron Man or Thor would be the third. "

i think the trinity are teams instead of individual hero's. the teams are: 
 
x-men 
avengers 
fantastic four 

I'd say Spider-Man, Captain America, and Wolverine.

Neither Wolverine nor Hulk seem to stick for me. They're not looked up to in the same way heroes like Captain America are. Anti-heroes usually aren't role models.