Steam – How to get Battlefield 3 to play nice as a Steam shortcut

As you already guessed, underslung attachments replace your primary kit, just like the normal grenade launcher would, which would be, as in the example of the assault class, the medkit.

You switch to your underslung attachment by pressing what you normally press for the medkit, you will then switch to your grenade launcher. You can verify that by observing the slightly raised barrel stance of your rifle.

If you haven't chosen the underslung rail as attachment option, then you will access to the grenade launcher as a separate gun, which will cause it to take longer to switch to and back.

For more information, take a look at this Wiki page.

The view of the classes you present in the question is way to simplistic.

Support

• Provides ammo. This is especially critical on big open maps with lot of armor. Most people play engineers using up their rockets really quickly;
• Having LMG with 3.4x + bipod, can provide accurate suppression fire at mid distance (upcoming patch is going to improve that);
• On rush maps can effectively suppress enemies preventing them from getting anywhere near m-coms. That can be done either with LMG + extended mags or with mortar (later is going to be nerfed in upcoming patch - 4.8s reload instead of 3.5s);
• Can run up to tanks and destroy them with C4, this is especially effective between buildings.

And BTW you say just "Recon has a sniper-rifle", which is big understatement.

Recon

• Can set spawn point, which is quite important on rush (well placed, can allow whole squad to spawn within defenders' base time after time);
• Can set SOFLAM, which greatly increases effectiveness of Javelins, also allows Javelins to shoot down aircraft;
• Can use MAV to spot enemies far behind their lines. Then Support can pick them out with mortar.
• Can set T-UGS near m-com, allowing defenders to react before attackers get chance to arm it.