Other mistake: During the revelation of the second death, when the killer opens the toxicology report they just retrieved, the date is revealed; it says it was issued on 09/27/2018. But the events of the movie happen in November (it's Sunday 18th Nov when Marta wakes up with all the reporters at her door). It should also be noted that when a close-up of the report was shown earlier (when Ransom was looking at the anonymous letter brought by Marta) that copy had a big stamp in a corner saying 11-17-18, that in this copy is totally absent. (01:56:55)
Other mistake: Marta sinks her white shoes in the mud to cover her tracks. We see that the shoes are, as they should be, all dirty to the sole and welt. They are clean again in the following scene though (on a sidenote, the shoes have been cleaned after the murder then, and she or her mom missed the bloodstain - it's very much possible, it just adds a little implausibility). (00:57:20)
Chosen answer: She is holding cake all 3 times, actually, so the only difference is that the first time she speaks. It can be a case of unreliable narrator, but I'd write it down as a mistake (deliberate, probably, as the phrase is important for the plot and they didn't want to hammer it in), since as you said, the other times even when they tell things to the police in a different way from what happened, the details tend to stay consistent, except for parts that are obviously made up, such as who is putting down the birthday cake for Harlan Thrombey therefore appearing subservient and not as close (when Richard tells the story, it's Walt and his wife, when Walt tells the story it is Richard and his wife).
Sammo ★