Remove the battery.
I think it is set to some other base pitch for a transposing instrument, like an E-flat clarinet, so while it is probably calibrated correctly, it is not displaying the same note names as on the piano. I don't think A=440 or A=450 or some other tuning is the issue. You want to get it back into "guitar" mode.
Usually the easiest way to re-initialize this kind of electronic device to its default mode is to remove the battery, wait a few minutes, and put the battery back in. Try this and see if it solves the problem.
I found a list of the features of this particular model of tuner. To wit:
- 3 Modes: Guitar, Bass, Wind instruments
- 4 selectable keys: C, F, Bb, Eb
- A4 calibration: from 430 to 450 Hz
The person asking the question said that the tuner is displaying 3 half-steps away from where it should be.
But A=415 is one half-step lower than A=440, and A=466 is one half-step higher than A=440. Note that this tuner can only accommodate from A=430 to A=450, which is less than a half-step in either direction. So this is clearly not what the person asking the question is experiencing.
Therefore, the problem is that the tuner is set to accommodate a transposing instrument in F, B-flat, or E-flat, like the list of features says. E-flat tuning is three half-steps transposed from C tuning.
The person asking the question needs to get his unit back to C and out of the Eb key transposition mode.
Here is how to do this:
Hold down the button labeled "Key".
Use the up and down arrow keys until the "Key" transposition indicator on the unit’s display, in the lower-right corner, says "C" and not "Eb" or something else.