One and six because of the configuration of the intake manifold would be my choice, but if this is the first time, I'd pull them all, but only after I had driven at least 250 miles, enough for whatever color has time to develop.
By pulling all you might see some differences among the plugs. If uniform amongst the set of 3 then you can feel safe in selecting one the next time....and there will be a next time, to represent the other 2.