First to answer your question, your castsequence will run through the sequence of spells and will reset 20 seconds after the last time you use the macro. Therefore, it's impossible for you to use this as a mashable ambush key.
The easiest way to do this would be this macro here:
Code:
/cast Premeditation
/cast Shadowstep
/cast Ambush
However, you will have to press it twice when premed and shadowstep are up to ambush as well. I personally just have a Premed macro'd to ambush and use shadowstep separately.