Well, this is just slightly different than the typical Round-robin, so there's nothing built in to help you exactly.
Start with the Mapped Key Wizard (from the Wizards menu at the top). First it asks for a Key Map to put the new Hotkey in -- select Combat. Then it asks the type of key to set up, select Straight Key Broadcast (because you just want it to hit the same key you press). Finally it will ask you what key, and where to send it. Tell it to send 3 to your first window (e.g. "Character Slot: 1"), make sure the other options look okay to you (whether it fires when you press or when you release the key, etc) and click Finish.
Now in the top left pane under Key Maps, select Combat. In the bottom left pane find the new "3" key it added, and expand it so you see Steps, 1. Click the 1. Now to the right you see Actions, and under that something like "3 -> 1". Right click Actions, select Keystroke Action (under Keystroke Actions), and fill it in to send whatever the other button is you want the other guy to press, and set the Target to the second window (e.g. "Character Slot: 2").
From there, you want something different to happen each time you press the Hotkey, so what you want is an additional Step for each time you press the Hotkey and want something different to happen. So probably 4 Steps. Right click on the first Step and select Copy Step to Clipboard. Then right click "Steps" and select Paste Step from Clipboard. This adds Step 2. Paste again until you have 4. Then click each added Step number (2, 3, 4), select the Actions to the right, and adjust the Targets to go to the next windows. So in Step 2, you send 3 -> 2 and ?? -> 3 (?? being your key for Laws of Justice), and in Step 3 you send 3 -> 3 and ?? -> 4, Step 4 3 -> 4 and ?? -> 1.
This completes your Round-robin as described.
Finally, you may need the same things to happen when you're holding Shift and press 3, since ISBoxer treats Shift+3 as a different Hotkey than 3 and will not activate the same thing. Right click on your "3" mapping you just set up in the bottom left pane and select "Create Alternate Hotkey". Then select "3 - Alternate" and change the Hotkey to Shift+3. Now whether or not you're holding Shift at the time, this round-robin should continue to operate as intended.
Don't forget to Export to Inner Space after making changes
