define Arg1 ebp+8
define Arg2 Arg1+4
define Arg3 Arg2+4 
    

Arg1 equ ebp+8
Arg2 equ Arg1+4
Arg3 equ Arg2+4    

But you might want to add brackets to ensure you get what you want because they are textual equates.

label Arg1 at ebp+8
label Arg2 at Arg1+4
label Arg3 at Arg2+4    

struc redefinable value
{
        local a
        label a at value
        . equ a
}

Arg1 redefinable ebp+8
Arg2 redefinable Arg1+4
Arg3 redefinable Arg2+4    

Code:

label Arg1 at ebp+8
label Arg2 at Arg1+4
label Arg3 at Arg2+4    

Pretty sure that won't work. A label is used to define a fixed value, a numerical constant. A function argument is based on ebp, which is a register (its value is only known at runtime, not at compile time), and thus is not a numerical constant. Using a label would work as a numerical constant, while what I need is a string constant, so that when it is compiled it converts that string constant into the literal string I want in the pre-process stage, and then the string I want gets assembled correctly in the assembly stage.