Line pairs equal twice the amount of lines.
For example 100 line pairs equal 200 lines.
Now we have to find out the width of one of these lines, when that amount of lines fills one millimeter = 1000 micrometers.
We do this by dividing 1000 micrometers by the number of lines.
Does a lens with 100 line pairs per millimeter support 3um large pixels?
This depends on the contrast that the MTF curve gives you for this pixel size and the respective application!
For monochrome applications, we can assume this 5um from above is the pixel diagonal. To get the pixel pitch, we can divide by the square root of two, i.e. divide by 1.41.
For color applications a typical value is half of this value, i.e. 5um / 2 = 2.5um.
For monochrome applications in this case 4.39um pixel (say, the answer is no) and for color applications 2.5um pixels are supported (= the answer is ‘yes’, if the MTF is good enough ).