3 -Man System

Positions at home, first (A) and third (D) - with no one on.
D is behind third, on the grass, in foul territory, just like A.
To set up based on where the runners are. A is in A all the time, unless there is a runner "ONLY" at second base - then A moves over to B.
D moves to C when runners are anywhere else, except if there is Only a runner at third, then D stays in D (remember, if a runner Only at second, A moves to B and D stays in D)
So, runner at first, first and second, first and third, bases loaded, second and third - A is in A, and D is in C Runner at third - A is in A, and D is in D
Runner at second - A is in B, and D is in D
We never want two umpires in the infield at the same time.
A will still move up to get a 90 degree angle on an infield play at first and D will move toward C
Runner at first, A up close about half way to watch for pick off and balk; runner only at third, D up close to watch for pick off an balk
on a base hit through the infield, D will move to C and A will float behind first (between first and dugout) and watch the touch and obstruction, and be prepared to cover home, but hang out for a possible run down at first.
On a fly ball, D will go out on any fly ball to center fielder over to left field line (this is the only time that A will come in and pivot and take the runner to second) - Plate covers third, ready to float back to home (A also, if Plate stuck at third) - so, A should take a quick peek at D - D is the key, if D goes out, right or wrong, A comes in and pivots.
On a fly ball to right field (to the right of center over to the right field line), A goes out, D moves to C, and plate watches the runner to first
It really is easy, but you will mess up because you are so used to Two-Man system

Think of it as a rope tied to your wastes, when one moves out, he pulls you in - always fill the gap. Never two umpire in the infield at the same time.

In 3 - Man, once you go out to the out field, stay out, and the system turns into a Two-Man