Currently, the ammobox and gear interfaces of ARMA 3 are awful compared to ARMA 2. I will not specific details, screenshots and videos below.
ARMA 2 Gear/Ammo interface:
All items in the box are a text list with small icons. Clicking on these highlights it and shows description underneath.
Arrows left or right of the items are used to take or drop weapons/items
The interface is split evenly - left side is ammo box contents, right side is your inventory.
Clicking on an item in your inventory will bring up and highlight the same item in the ammo box. This is good because it allows quick rearming.
The UI is easy to understand and contrasts well with any background due to limited transparency and good colour use.
The UI is self-contained and doesn't have any gaps or wasted space.
Now... on to ARMA 3's gear interface. It sucks, because:
The list of items of the left has icons that are too large, and the text spans multiple lines. This is an unclean/messy waste of space.
There's a big gap between the ammo box and your personal inventory. Why? It has no obvious purpose.
Clicking on an item in your inventory doesn't bring it up in the ammo box - you have to scroll through the large, blocky items with poor text readability to find the item you want.
There's lots of wasted space in relation to icon sizes for your personal inventory. The handgun icon is only so large, but it takes up the same amount of UI space as a launcher or rifle. Why? It's a waste of space.
Double clicking items to drop/add them to your inventory doesn't work (this has moved to right click, I know)
The filter function has changed from a single click option to change category, to a double click function and a drop down menu. Overall it takes more time/effort (and mouse clicks) to perform the same function.
There's no drop/add arrows in the ammo box to quickly add or remove items from your inventory. This is partly because you can now have three different backpacks, but taking weapons should put them in a weapon slot automatically, and backpack if the weapon slot is taken (but why would you need to carry more than one weapon anyway? Addons solve this problem already)
This applies to all items. The add/drop buttons in the ammo box missing means that it takes more time to sort out your inventory in general.
Please see these two screenshots and video linked below.
ARMA 2: http://i.imgur.com/ekld3WS.jpg
ARMA 3: http://i.imgur.com/xaM1Xd1.jpg
Video Comparison: http://www.youtube.com/watch?v=ohsi6dxt8wY