Yes - on two packs a day you'll be on less than 300 calories and not sufficient nutrients - your body will start hanging onto fat and burning muscle instead, which is exactly the opposite of what you want it to do. Your weight loss will slow down and you will feel ill.
Please have all your packs - it's very, very important. Even if you have to hold your nose and glug it down like medicine.
I actually went to bed before I realised I had not had my third pack. I promptly had a mint hot chocolate as a bedtime drink. I didn't want the loss of nutrients to affect me health wise or my diet as a consequence.
I agree with everyone, do take all three packs. Good luck on your journey.