<em>The word 'linear' means something having to do with a line.On a Cartesian Plane, a linear function is a function where the graph is a straight line. The line can go in any direction, but it's always a straight line. A non-linear function has a shape that is not a straight line.</em>
You have 8 choices for the third digit (digits 0 - 9 excluding the numbers you chose for the first and second digit). So by the multiplication principle (MP), there are 9*9*8 ways to do this. So there are 648 numbers that are 3 digit numbers and have no repeated digits.