The first thing that comes to my mind is that text field adds unnecessary cruft while being needed in only one situation. Consider showing this field only when "Constant" is selected. Once "Constant" is selected, it might be a good idea to focus the associated text field, so that the user doesn't have to work to target the newly appeared field.
To comment on your variants:
I don't see the value of typing in a choice -- that will always be less efficient that selecting one of three choices. A simple drop-down menu might be better.
The combo-box drop-down is still be inefficient, though. It's two clicks per choice, but I'm oversimplifying the problem in this case (it's never just about the number of clicks). Consider that the drop-down will cover the next choice(s), so after it's closed, the user has to process where the next drop-down is before targeting it.
The yes/no states are clear, but the "constant" choice is not guessable. While you could theoretically explain it away in a description, it would still not be "intuitive" -- it works against one's standard mental model of how a check box works.
It's less efficient than your radio button proposal.