Spidey Wolverine and Captain America are Marvel's trinity.
They're the ones with the merchandising, the lego toy lines, the knex figures, etc. T Shirts at wal mart.
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The only real way to answer the question, is to figure out what is meant by "Trinity". If we define it as the three highest money makers of the company or the three most well known public figures, then it would indeed be Spidey, Wolvie, and Hulk.
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For a slightly different take on this, who would compose Marvel's Trinity if you tried only to use characters who are analogous to DC's?
For Superman (symbol), I'd have to go with Captain America. Similar "boyscout" type perception of them by most other characters and seem to inspire loyalty in most other heroes to some degree.
For Wonder Woman (warrior), Thor is an easy pick. Not only are they both outsiders in a sense and magic in general, but both are also respected for their combat abilities.
For Batman (unknown), I'd go a little bit out of left field and say Dr. Strange. Both are essentially human and have gained their powers through their own efforts and "gadgets." More importantly they seem to operate more independently and separate themselves from other heroes to some degree. Also they seem to inspire a combination of respect and "spookiness" (if that's the right word) in others.
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