First, from the description it is clear that these are not three similar characters. In fact, there are two MCs, and another character. The question is "how to make the reader feel that the third character is equally present in the story, and not just popping out here and there?""
There are three cases I can think of:
- the three characters are roughly equivalent (e.g. three hobbits), and the issue is that you have not fully outlined the motivations of the third one.
- the third character is different from the other two (e.g. two hobbits and an orc), and the issue is that
- the third character is Jar Jar...
I think that the issue could be that you find your third character not-relatable.
This perhaps is due to the fact that you don't know what truly drives them. Their actions are justified as a whim, and taken without much of a thought. Imagine now that you had to put yourself into their mind and justify the truest reasons of their actions. "Out of a whim" is not a justification. There needs to be a stream of thought in their mind that went from "external cues" to "action".
Taking Pippin as an example, one may find him funny, clumsy, lighthearted. Now imagine living all your life under such a mask. Perhaps you wished to do something worth praise and never quite succeeding because you believe to be clumsy, or not clever enough. Pippin may laugh at himself, and may play the fool to fill his role in the hobbits' society, but perhaps, deep inside himself, he just wished for a bit of respect. In fact, by the end of the Lord of the Rings, Pippin is entirely another person, with stature and dignity, and not one in Gondor would lightly say of him that he is a fool.
It seem that you have quite understood and outlined your other two characters. You need to work on this third one. Place three people at the same starting point. What would you need to push one, and only one of them, to be like your third character? Considering that the three characters are interchangeable in this scenario, it is fair to imagine that the third one has a longing for a more normal/respectful existence, even if it does not look like that, even if they seem so lighthearted.
It does not need to explode in a conflict with the other two, but it will serve to give context to the reader to related to your third character. Anyone can relate to a jest, just bring forward the reasoning.
Not the best example, but you can go from telling about the crazy jests:
He covered himself in white paint, and danced across the room, like a chicken. "Look, I am a swan!" said Porthos. "You are making a mess." laughed Athos. They ran after him with Aramis.
To showing the inner thoughts that lead to them:
He covered himself in white paint, and danced across the room, like a chicken. "I'm a beautiful, beautiful swan." he said. "No you're not. You're just making a mess." laughed Athos. "Yes, I'm a swan. And if I am not, then I want to be a swan! You two are what you want, why can't I be what I want?" insisted Porthos flapping his arms.
Case 2. If the third character is a completely different nature compared to the other two, then the true issue is that the other two are not interacting in a relatable manner to the third character.
A third character that is different in nature to the other two will have a constant presence in all their actions, all their conversations, and all their activities. Nothing can be said or done without taking into account the presence of the third. It is a looming presence. It does not need to be negative, but it can't ever be ignored.
Think of it as two elephants hanging around with a zebra. The zebra may do things that look funny, or strange, but, "hey, it is our friend the zebra". They want to go for a swim? Discuss to check if the water is ok for the zebra. They want to grasp fruits from the tree? Discuss to pass some fruits to the zebra. Want to do X? Discuss how zebra will react.
It will not be on a narrative equal level, and from the question it is clear that it is not, but it has to become equal in terms of presence. Zebra can do all the crazy things zebras do, but the readers are now ready for it, and, actually, they will expect it.
No, there is no case 3.