What is the difference between comp and comp-3 usage ?

Comp :

COMP is 2 bytes.
COMP is used for Binary Representation it allows only S and 9.
COMP stores the data in the binary format.
With COMP fields, you have to drag out a calculator and convert the hex value to find out the true value that is being used in a program.
Memory usage is,

COMP-3 is 8 bytes.
Comp-3 is used for Packed Decimal values it allows S 9 V mostly it is useful for Decimal Calculation Values.
COMP-3 stores the data in the BCD(Binary Coded Decimal)format.
It is used to avoid rounding issues that occur when binary numbers are used to represent decimal fractions.
This is particularly vital in systems that method massive volumes of economic transactions/amounts
Memory usage is

The usage clause specifies how the data item being stored internally .If no usage clause specified , the default usage clause would Usage is display