You can mix and match if you want however, I would recommend that you attempt to complete pullups first, pushups second, and squats third. The reason for this is that as you progress you will want to be able to pump out more than just a few pullups per round. Especially in the pullup department because once you get in a rhythm you can complete more in a faster period of time. Day 3 is a great way to develop that. As I've progressed I attempted to keep to that thought process as much as possible and I think it has helped. Although I admit I didn't always do it.
"Permanence, perseverance and persistence in spite of all obstacles, discouragement, and impossibilities: It is this, that in all things distinguishes the strong soul from the weak”