How to determine the grain of a fresh/ uncooked Arm Roast Newbie.

I have a 3-lb fresh Arm Roast from a local butcher shop.
They did not slice it into 1-lb. slices this time, so I need to figure out how to do that. 

It looks like my first obstacle is figuring out the run of the grain structure and then which way to slice the meat, with the grain or against the grain.

The second obstacle will then become how to slice it into roughly three 1-lb slices. Given its shape (paraboloid) stirs distant memories from geometry class! More math than I want to take on right now for making two cuts through it. 

So I am thinking that there must be some rule of thumb for making two such cuts through the roast.

Alternately I thought I might come close by starting at the halfway point then measuring out a half an inch either way, getting a 1-inch steak from the center and and two mini-roasts. I would weigh all three sections for future reference.

Any thoughts will be greatly appreciated. I do need to get this roast into the freezer tomorrow, even if I have to freeze it uncut.
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.