As stated in our Antagonist fussing policy, labels like "deuteragonist" and "tritagonist" should not be used due to the numerous edit wars that have occurred when people fight over trying to rigidly define how much of an antagonist or protagonist a character is. Please do not add these any more.
You're right with them being the second and third main characters. However, the deuteragonst is the second most important character, while the tritagonist is the third most important. Meaning, Sherman is the deuteragonist, and Penny is the tritagonist. Don't you think it'd be weird if Sherman was the third most important character, when his name is in the title of the movie?
What I have seen is that when people start lining up characters side by side and pinning labels on them to say "This one is most important, that one second-most important, that one is third-most important, that one is fourth-most important, that one is fifth-most important, that one is sixth-most important" and so on, and so on, and so on, that's ALL those people tend to do. They get focused on trying to rigidly-define exactly how much of a good guy or a bad guy the character is that they ignore everything else. In addition, there is always at least one other person who will have a different rigidly-defined order, so then you wind up with an edit war because each person is trying to make sure their rigidly-defined order is kept in place.
To put it blunly, labels like "deuteragonist" and "tritagonist" say nothing about who the character is, what they do, how they relate to others, what their motivations are and everything else that makes a character someone the audience would be interested in. It's why I created a policy to prohibit antagonist/protagonist fussing.
I'll ask both of you this: Do you line up your friends and point to one and say, "You are my primary friend", then point to the next one and say, "You are my secondary friend", then point to the next one and say, "You are my tertiary friend", then point to all the rest of your friends and define them as being fourth-level friends, fifth-level friends, sixth-level friends, seventh-level friends, and so on, and so on, and so on?
I really hope you don't. If I was your friend and you did that to me, I'd be insulted that you thought of me that way. To see you say, "RRabbit42 is less of a friend than this other person" is not the way you build a friendship. You build friendships by the things you experience together, the details about your lives that you share, what you like, what you dislike and so much more.
So don't do that in the characters in movies and TV shows. Stop trying to rigidly-define exactly how much of a character they are. Look at all the details about that character's life and focus on those.