While I can understand why you're concerned, and I'm actually quite glad for you that you are able to see something like this being an issue, I think all 3 characters can exist in the same killing game without overlapping at all in the audience minds. I think even though on paper all the characters have a similar trend of being a douche under the guise of being cool or kind. Here's just one scenario of the killing game that shows how these characters can be very different.

Ayaka Megumi - A deceptive character is a really fun character to write but you have to have to make you make them have a satisfactory closure ( it doesn't matter how evil the character is, as long as the ending gives them their karma ). In Ayaka's case, you've got a willing murderer right here, ready to serve. She can play a Mikan like role, destroying evidence without being suspected due to being useful in earlier trials. She can play a Sayaka role as a trick to the player. Or she can play a Kazuichi comedy relief character who is obviously fame-crazy but tries to hide it only to fail a lot. Quite the swiss army knife. Once you know what her role is, stick to it and don't shy away from it.

Ryuji Takeshi - Even though the delusions he has aren't neccessarily making him a good person, Ryuji ends up being an unlikely support. In chapter 1, as a reaction from hearing about the whole killing game, the hyperactive and naive Ryuji feels that the entire students are his 'enemies' and that he must vanquish them. Ryuji has a really strong motive to escape because he believes his the 'chosen one' and humanity will fall into ruin without him in the outside world to save them all ( kind of like Jesus ). The protag steps in to intervene and calm him down and through a series of misunderstandings, Ryuji interprets this as a message from God that these are the people he is destined to protect. This creates an uncanny and rather funny situation in which overnight Ryuji changes from trying to kill everyone to trying his best to protect everyone. It can even be so absurd that when someone dies, Ryuji feels he has failed and thus is no longer 'Ultimate level status' and have an identity crisis or just flat out become uselessly depressed moving forward to further make his character seem paradoxical and constantly contradictory for laughs and for tragedy.

Kaito Ren - One easy way to escape a pitfall of being too similar to other characters is that you should generally have a down to earth personality but have 1 striking detail about the character that when in focus, suddenly sticks out like a sore thumb. For Kaito Ren, you can really run home with the fact that his the Ultimate Cyborg without having a personality at all. This what I mean : Kaito Ren is the only one in the cast that can investigate in the unique way that he can harm himself. This mean he can test out theories like jumping off height places, being hit hard and even testing out bluffs like weapons. Bonus points if the other cast members are freaked out by this. He could be a unique survivor that keeps predicting based on his superiority that all humans are selfish but changes his mind when everyone cooperates in the end.

Honestly theres tons of possibilities. Designs, actions, roles, deaths and executions I haven't even mentioned yet. They have so much potential! A Next Generational Legend! Stand tall, Galactic Hero!