I think you only need two different calibrations, not all 3. Either 7 and 10, or 7 and 4. That way, you're calibrating neutral pH (7), and a variable. I've always used 7 and 10 because the range we shoot for in our tanks is between 7 and 10. I think I've read this about 2 point calibrating before, but I can't remember where. Not like this saves you a while bunch of money, but every little bit helps.