When I browse this forum today, one can't help reading the I-824 arguments, so I was intrigued by it and went the distance to check on it myself. The following is my conclusion:
1. Part 3 is for yourself, not for your spouse and/or kid(s) back in China. It's all in the instruction. Period.
2. The I-824 forms of Spouse and/or kid(s) back in China have to be filed after your I-485 is approved. Period.
3. In case you want to do both I-485 and CP, by law, xiaobaitu is right. When you apply I-824 as 485 is pending, your 485 would be canceled in order to continue CP.
However, in practice, some applicants did both CP and 485. The reason is it took too long for USCIS to process the I-824 form, and some U.S. consulates would agree to process the CP based on an attorney certified copy of the I-140 petition and the receipt notice for the I-824.