I have a head canon that captain 3/original 3 is particularly mute and can only talk to people directly once they're really comfortable with them. This explains why in the story mode of Splatoon 3 When you go talk to Captain 3 Marie speaks for them. They don't know new 3 very well yet so they physically can't talk directly to them like they can with Marie.
Similarly, I imagine 3 is closer with Marie very slightly. Hence why it's almost never Callie telling you what captain 3 said. Though they can and do speak. Callie just sounds more like c3 in delivery of sentences so they Don't have to worry about being misinterpreted (I imagine that happened a lot before they figured out how to speak with people directly.)
Smaller similar head cannons:
C3 has a text to speech thing that the hardly use because most of the time they view words as unnecessarily complicated.
C3 has gone a full year without talking to anyone and they were thriving.
8 didn't speak inkling yet when Craig met them. Still didn't fully by the end of it but Craig knows really rusty old octoling from before the war and 8 cleaned up some of his dialect. Turns out that inkling and octoling were alot more similar before the war but post war octoling is a lot harder to translate to inkling and visa versa.
This last bit is a theory
Forgive me for not remembering the name but the printer thingy that does splatfests is an old Nintendo console that is vomiting out the splatfest themes from our Splatoon games now to the future inklings and octoling's If that makes sense. This explains how Splatoon one's final splatfest could have been on that printer thing if it was from the humans.
