Perfect three-part messages are composed entirely of things about which no rational person could argue. [If the person you're interacting with isn't rational, that's a different problem, and beyond the scope of this brief post.] My favorite perfect example is this one, already familiar to you:
"When you don't water the tomatoes, I feel angry, because plants die without water."
Part one ("When you don't water the tomatoes....") is the one specific behavior you want changed. Unless you're also irrational you won't be saying that unless the person really has failed to water the tomatoes. It's verifiable in the real world, and it contains no judgmental or subjective chunks of the sort that "When you don't do your fair share of the garden work" would. It's a valid part one.
Part two ("I feel angry...") is the weakest part of the three-part message in terms of the requisite real-world verifiability. English allows dialogues in which one person says "I feel tired/sick/angry/whatever" and another person can acceptably, although rudely, say back "Oh, you do not!" When properly constructed, however, it's verifiable in the sense that it is (a) appropriate for the context in which it's spoken, and (b) backed up by the speaker's body language, including intonation and tone of voice. It's conceivable, though unlikely, that it may be challenged with "Oh, you do not feel angry!"; if that happens, the appropriate response is just to repeat the original three-part message even more sincerely.
Part three ("because plants die without water") is the hardest part of the three-part message to construct. Its function is to say why it's your perception that you are justified in making the request for a change in behavior. To be valid, it must also be concrete, verifiable in the real world, and not subject to rational argument. "Plants die without water" meets those specifications. None of the following alternatives would qualify:
"....because it's not fair for you to leave all the work for other people."
"....because you're just being lazy."
"....because a decent person would do their share of the chores."
Any of the three unacceptable alternatives might well be true, and justified. That's irrelevant. They don't qualify as a valid part three for a three-part message.
There will be lots of times when you want a behavior change from someone and you find that you can't put together a valid three-part message, usually because you can't construct a valid part three. In such cases, you'll have to use some other technique. When a three-part message is available, however, it's the quickest and most efficient way I know to get the task done.
The only wording change that I can recommend is -- if you consider it appropriate and/or necessary -- a shift from Leveling Mode to Computing Mode in part two. Like this:
"When you don't water the tomatoes, people feel angry, because plants die without water."
This is an instance when your communication goal is the deciding factor. If what you want to do is tell the offending person how you feel and get that off your chest, it doesn't make any difference what you say as long as it accomplishes that goal for you. If what you want to do is raise the offending person's consciousness, you'll go about it differently. If what you want to do is persuade the offending person to change the one behavior in question, the three-part message is ideal, and should -- in my opinion -- be the first thing you try.