Hi i want to declare a datatype of size 3 bytes

[anilchilli]

I dont want to use the standard datatype
can any body help me how to declare my own datatypes of my
own choice.

[ashitpro]

You can have a structure and declare the char array of size 3.
Or best way to create a class, here you can also define custom methods to perform operations on your char array

[Banfa]

However you would be far better off just using a 4 byte integer and range checking it before it is used. It is unlikely you will save enough memory by having a 3 byte rather than a 4 byte int to make ashitpro suggestions worth while.

[weaknessforcats]

What is this 3 byte datatype supposed to look like?

You use struct or class to declare your own datatypes.

[Banfa]

Actually in fairness I have worked with compilers that had 3 byte ints you declared them

long short
or
short long

However as a note to the OP that was most definitely a compiler extension and caused not end of grief when I determined that the clients existing code base was written to a poor standard and attempted to use a static analysis tool on it.