Reverse number value from positive to negative

Hi

I have a csv file and where I need to change the value from a positive to a negative and vice versa where a column is a certain name.

Col 1      Col 2      Col 3

CT         Date        256.66

CT         Date        -0.26

In the above example in first row where col 1 = CT I need Col 3 to be -256.66 and row 2 Col 3 will need to be 0.26

Any assistance will be good.

Chris