In game macros cannot change targets within itself beyond using help or harm. At least, not in the way you describe.

You can use the Toggle command in HKN to set up the roundrobin and avoid using macros altogether by calling a target command on downpress and the spell on release

Code:
<Hotkey F3>
     <Toggle>
          <SendLabel w2>
               <Key F1>
     <Toggle>
          <SendLabel w2>
               <Key F2>
     <Toggle>
          <SendLabel w2>
               <Key F3>	
		
<Hotkeyup F3>
     <SendLabel w2>
          <Key 3>
Of course, you need to make sure you *don't* spam it, otherwise you'll lose track of where the toggle is. You can always use <ResetToggles> somewhere to put them back at the first one.