It looks like and DERIV are the same thing. You need to give it an expression and a variable; it then takes the partial derivative of the expression with respect to the variable. The third one, DERIVX, is the same thing except it assumes your variable is x, so you only need to give it an expression. It is true that when you have a function of only x, the partial derivative with respect to x is the same as the derivative with respect to x.
Try taking the derivative of something like using all three methods and see what happens.