How can you view the durability? When I do F3+H it doesn't work. I have version 1.7.2, and I'm using mods, but not any durability mods. I'm using a Lenovo laptop and an Acer laptop running Windows 8.
Most Lenovo laptops I've seen have certain function keys imposed on their F-keys. Mine does.
For example, is your volume, brightness, mute, etc. on the same keys labelled as F keys?
If so, try pressing FN + F3. (Your FN key is the key between CTRL and Start/the Windows symbol key.)
Basically, think of FN as a shift button for your F keys and a few others.
For people not using a laptop and just using a Windows PC, the best solution for this is to press F3+H. This will add durability for weapons to your UI. If you are using Mac or a laptop you need to press FN+F3+H.
This is the best way to display durability in vanilla Minecraft.
Mods are available: