What the difference in data if calibrate pH-meter with 2 and 3 solutions?
You cannot &quot;predict&quot; any error by thinking that if we calibrate with two buffers the error will be x, and if we calibrate with three buffers the error will be y.
Ideally, in each case, the pH of the sample should be identical. Good quality pH meters tell you how close they are to the theoretical slope in terms of percentage. It should be close to 100%.
For example, at 25°C the theoretical slope is ~ -59 mV/pH. If your calibrate with two buffers and  is close to -58 mV/pH, the slope percentage is 98.3%. It is good to go. As electrode age, the percentages goes down.
As EdV said, three points might be better for the $12 pH meter.