Are they using the same interface guy/gal this time? That might determine my decision to purchase.
This answer is from last year so things might've changed since then:
How has the UI been improved? I found Arma 2 to have one of the most needlessly byzantine interfaces in recent memory. (@hhjanes)
The action menu as we know it (using the scroll wheel to do everything from entering vehicles to grabbing ammo and phoning in airstrikes) is in tact. Bohemia says this system could still change, though.
What has been revamped, thank goodness, is player inventory. The version of the gear menu shown wasn’t final, but . A left-side column of the menu represents what’s on the ground, with the right ? of the screen separately showing what’s on your person. That’s already an improvement over having the items mixed together, as it is in A2. There’s also discrete spaces that show the type of vest you’re wearing and the type of backpack you’re carrying, and their encumberance levels.
Removing a scope from a weapon (or reattaching it) was as simple as hitting G, double-clicking the scope, and hitting G again to close the menu. The response time of the inventory was much better, too. I didn’t have to wait a full second for a magazine or weapon to transfer.