A bull moose is nowhere near as hard to kill with an arrow as a bull elk. Moose go down easy.
If you hit one lung on a moose, you are likely to kill him. If you hit one lung on a bull elk, you have big, big trouble. You may find him, or you might not. But you almost certainly will not find him close.
Unless you are shooting a heavy bow (65 or more), you would be wise to get the extra penetration of a two-blade.
Yes, you CAN kill a bull elk with a three blade, and I have done it myself, but then again, my lightest bow is 75 pounds. Tip the odds a little more in your favor by going for deeper penetration.
Remember-----bull elk are the luckiest critters in the woods, and if they can make something go wrong for you, they will.