You mean you are going to break with lemon water? If yes, the answer is yes you can. Do you want to continue with lemon water for 21 days? The answer to that is to take a day at a time. When you feel weak you have to to start eating fruits. I searched on CZ and did not find anyone who has done Master-Cleanse after fasting.
All in all you can do it but at first sign of weakness or exhaustion please eat fruits and slowly build up your food intake.