How to determine the grain of a fresh/ uncooked Arm Roast
For most cuts of meat, the direction of the grain should be relatively easy to see. Usually at least one face will have no visible striation, and the grain will run roughly perpendicular to that. If you scrape at a bit of fat or connective tissue at the surface with the tip of a knife, you can see what direction it takes down into the meat. And if there's a (long) bone, the grain will usually run parallel to it.

As for dividing it up evenly, that just takes practice. If you really want things exact, though, and you have a pot which you can submerge the entire roast in, you can use water displacement. Put the roast in the pot, and cover with water. (If the roast floats, keep it down with a thin metal utensil.) Note or mark how high up the water is in the pot. Then remove the roast while leaving the water in, and check or mark the new water line. Find the level 1/3 of the way between the two lines, and dip the roast down until the water reaches that point; note where on the roast the water comes to, and cut along that plane.