We have a 3-year-old who does something similar at the moment. She doesn’t put *everything* in her mouth, but when she’s upset with us, she will grab the very thing she knows is worst and put that in her mouth—whether it’s Daddy’s laptop plug, or a disgusting leaf the dog brought in from outside, or, well, you get the picture. We’ve tried talking to her about it and she will promise not to do it again, and then promptly do it. So far the only thing I’ve found that works—and this may not work for you because you have 2 others and it may be unfair to them—is to abort whatever activity is going on and explain that we simply can’t do it while she keeps putting things in her mouth. I know this sounds suspiciously close to punishment, but we don’t remove her to another room or even make her get off our laps if that’s where she is; we just stop what we’re doing. For instance, yesterday I was reading her a new book and she reached out, grabbed the bookmark that had come with the book, and stuffed it in her mouth. I took it out of her mouth and told her that if it happened again, we’d stop reading; of course it happened again, so I got up, put away the book, and just sat there with her. But I can easily see this having no effect on another child with a different temperament—Ru absolutely hates being interrupted in what she’s doing, and yesterday she was OUTRAGED that I wasn’t going to read to her anymore. I didn’t mind the outrage because it made it possible for us to have a conversation about what was happening, what she had done, why it wasn’t a good thing to do, etc. etc.
Sorry I haven’t any better suggestions, but I’m waiting with bated breath to see what more experienced mums will suggest!