Things you could improve:

Bugs

• Your initial printing of the input command line arguments looks buggy.

cout<<argv[1]<<endl;
cout<<argv[2]<<endl;
cout<<argv[2]<<endl;

It looks like you meant to print argv[3] here, but accidentally put a 2.

Syntax/Styling

• Please don't use using namespace std;. It's considered a bad practice, and is a bad habit to form.

• Use std::isdigit() instead of your isnum() function.

• You don't need to include the <cstdlib> or <string> headers (you will have to include <string> if you implement std::isdigit()).

• Put the variable declarations to separate lines.

double start,end,step_size;

From Code Complete, 2d Edition, p. 759:

With statements on their own lines, the code reads from top to bottom, instead of top to bottom and left to right. When you’re looking for a specific line of code, your eye should be able to follow the left margin of the code. It shouldn’t have to dip into each and every line just because a single line might contain two statements.

Loops

• You could use more loops throughout your program.

if(isnum(argv[1]) && isnum(argv[2]) && isnum(argv[3]))
{   
    cout<<argv[1]<<endl;
    cout<<argv[2]<<endl;
    cout<<argv[2]<<endl;
    ...

Using a simple for loop here to reduce the code down a bit.

    for (int i = 0; i < 3; i++)
    {
        if (std::isdigit(*argv[i])) std::cout << argv[i] << std::endl;
        else return -1; // or re-ask for user input
    }