This is more common than you think. You know 3 languages with a lot of common vocabulary, so it is inevitable you might mix things up. I don’t think it would happen if you were learning Thai or Swahili, or happen as much. The brain has only one centre for languages, so this also explains the confusion.
I think the way around this is reading in your target language but writing it out as well. In effect you write out in advance what you want to say.
I think there is a big leap in learning when you write, as this makes things clearer, and then you can speak without as much confusion.