Set up a variable that you add to. Press down(+1), press right(+1), press z(+1). Start a timer of 2 seconds upon pressing down. If variable = 3 within 2 seconds then perform attack. Logic will be something like :
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On Down Pressed
Start 2 second timer
+1 to Counter
On Right Pressed (If Counter=1)
+1 to Counter
On Z Pressed (If Counter=2)
+1 to Counter
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Every Tick (If Counter=3) > Perform Attack
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On Timer Ending > Counter=0
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That is how I would do it and it should work.