The words "tritagonist" and "minor character" don't mean the same thing at all. Tritagonists are characters who are third in importance in their stories, and minor characters have little importance. So Penny Peterson is the deuteragonist/tritagonist of Mr. Peabody & Sherman, and that's the final decision.
Actually, it's not because for some people, tritagonist means what you said and for others it means there are three equally-important characters. And then there's people who look at overall importance in the story and others factor in whether the importance increases or decreases during the story.
Nobody can agree what definition to use and it gets worse when someone decides to modify it by putting "secondary" or "tertiary" in front of it. A "secondary tritagonist" then becomes either "this is the third-most important character but they're actually less important than that" or it's "there's three equally-important characters but they're not as important than the other two, which really should be called deuteragonists".
You can see more reasons why trying to precisely define how much of a protagonist or antagonist a character is becomes very problematic in this message. Look more at what makes a character a person and less at what label they should be assigned.
I personally believe that the -agonist terms (with the possible exception of "antagonist", although some people erroneously believe that to mean "villain" rather than its proper meaning of "opponent") should be banned from this wiki, because of their ambiguity and consequent misuse. Some people even believe that "protagonist" means "good guy" (rather than "main character"); how do such people explain Felonius Gru, the protagonist of the first Despicable Me movie?
Greeting I'm Landyn Gunderfan and I love the picture you put in the New York Page, Not to brag but I've known most of the buildings on the NYC Skyline since 2017, the obsession started in 2013. Fun Fact I'm in charge of the New York City Wiki on the image I can see The Empire State Building, One Bryant Park, The Hampshire Hotel, Javitis Offices, and The Plaza hotel what's the image from