Question: immediate evaluation and dealyed evaluation

I need to evaluate a variable inside the function definition in the moment it is defined and not every time it is called. Is there a way around?

f := x-> a*x;


I want the function "f" to be 3*x even if "a" is modified after the declaration. In Mathematica I am used to the so called immediate assignmet(=) and delayed assignment(:=) for which I cannot find an equivalent command in Maple. 

Any help is appreciated!

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