How do you estimate 640 3

Well it depends on what ... tens, ones, hundreds, thousands.

and is that 6403?

Depends on what your rounding, are you rounding to the nearest tens, hundreds, or thousands? If its tens, you round 640 to 640 (it stays the same) and if its hundreds you round 640 to 600, and if its thousands, you round 640 to 1,000 ... (P. s I might be getting some of this wrong)