Is the name "Chell", "GLaDOS", or "Wheatley" ever explicitly stated?

Names are very rarely used in the Portal series (aside from Cave Johnson and his narcissism complex, and Caroline), and you are correct in none of the names referenced being spoken in-game (Per the Half Life Wikia):

Interestingly, neither the full name of GLaDOS or her acronym has ever been pronounced by any characters in the games. (Except for Cave Johnson in the DLC Perpetual Testing Initiative, where he says "we are cancelling the Genetic Lifeform and Disc Operating System initiative.") The only instances of her name's presence was through her captions, the acronym spotted on her disc generators in Portal, and the door within the trap leading to the Central AI Chamber in Portal 2.

However, GLaDOS says "Chell" exactly one time...in an audioclip never used in the final version of Portal 2:

Additionally, in the credits song of Portal 1, GLaDOS clearly sings "I'm so GLaD I'm not you" with the lyrics having that exact capitalization, also hinting towards the name GLaDOS.

The names are probably simply not mentioned for immersion reasons--the player is simply supposed to be Chell, and how often do friends call you by your name in one-on-one conversation? It's less clear why GLaDOS and Wheatley never speak their names (and they sure don't), but if you leave subtitles on it's quite clear what the character's canon names are.

Just to add to the existing answers, the credits in Portal and Portal 2 do actually give explicit names.

From Portal:

From Portal 2:

In Portal 2, when you're in the employee day care, the mutant potato science project has "by Chell" written on it. Whether this is the same Chell is up for debate, but this points to the possibility that Chell is the daughter of an Aperture employee.

In one of the Perpetual Testing Initiative tests, Cave says "the Genetic Lifeform and Disk Operating System" (GLaDOS), but other than that, GLaDOS' name is not spoken in either the shortened acronym form or in full (by characters in the game). Her name can, however be found in the second game on the fake door in Chapter 5 that you see just before entering GLaDOS' chamber and in an information icon if you wait long enough before picking her up at the end of Chapter 6. Wheatley says his name once, and Chell's name can be found in the corner of one of the "Bring Your Daughter to Work Day" projects. Both GLaDOS and Wheatley's names can be found if you turn the closed captions on, and all three of them can be seen in the credits and heard spoken if you play through it with developer commentary.

At one point wheatly after his takeover does at one point sarcastically mention how "scared" he is of "chell and a potato"

the name chell is from the comic book series "Lab Rats" which also tells the story of Ratman and the talking Companion Cube.

GLaDOS's name is not spoken in game but it is in text (aside from closed captioning). The sign on the fake door before GLaDOS drops you into the moving glass room says "GLaDOS Emergency Shutdown and Cake Dispensary"