[cbfoulds]

First of all [and no disrespect intended to you], let me put on record that I hold the concept of "1-man mechanics" to be an oxymoronic joke. With only one umpire on a 90-foot field, you are, of necessity, making s#!t up as you go, mechanics-wise. Get where you need to be to see as much of what you need to see as possible, while remaining in a sufficiently flexible position to react to events as they develop. And take ABSOLUTELY NO static from the cheap SOB coaches who won't hire a second set of eyes and legs.

-<[/Rant off]>-

That said, I have the following suggestions:

A] contrary to all your instincts and proper TWO-MAN mechanics, react to this sitch by moving INTO the infield toward the home-side of the mound. From here you have improved your distance toward the likely 1st play at 2d base, and also improved your angle for following plays on either corner. 'Course, if they try to cut down R3 at the plate, you may eat horsehide for supper, but at least you will be looking into the play at the plate [assuming you are not wearing the ball], rather than up its azz - which is where you'll be if you go up 3bl in foul territory. AND, moving in this direction allows you to "open your field of view" more than [realistically] you can in foul territory, increasing the chance that you will see R3 cross the plate, or at least know how far away he was when you turned your attention exclusively to the developing 3d-out play.

B] Rather than not counting the run because you are not SURE R3 scored in time, I would advocate the opposite analysis: score the run unless you are SURE the 3d out came first. By all means, use the techniques and secondary evidence suggested by others to "catch" the R3 who lollygags his way down the baseline; but in most cases, R3 is off like a shot and hauling freight, with a substantial lead-off into the bargain. If R3 DIDN'T score in time, he probably tripped over his shoelace and face-planted short of home plate, where you will find him when you "GLANCE at the Runner(s)" after "watch(ing) the ball". If when you look back at the plate after calling the third out, R3 is halfway to his dugout high-fiveing his teammates, it is very likely he was across the plate before the 3d out was made.