As with the other classes, here are some tips I have for scoring the 3 SPECACT awards with the Medic’s MG3 SA.
The MG3 has the fastest fire rate of all LMGs; it eat bullets at an absurd rate. Unless you’re a very conservative player, you’ll find that you are emptying your magazine faster than you can say "noob tube". That rate of fire combined with the 100 round magazine count (200 spare for two reloads), you will be wasting a lot of bullets unless you are careful. To counter the insane fire rate that tears any unlucky foe to shreds, it’s firepower is very weak; more or less the lowest damage of all the LMGs. This would make the Magnum Ammo perk seem ideal, but I personally see otherwise. Since you are going to put more bullets into your enemy than you intend to, having extra damage is overkill and impractical. Thus, equipping Marksman LMG Training to lower the insane spread of the MG3 is very wise and practical. However, if you take Marskman training then do not take Red Dot or 4x Scope. The reason is that Red Dot and 4x apply their own modifications to fire spread, both of them override Marksman LMG Training, making it wasted. If you want Red Dot or 4x then take Body Armor or Magnum Ammo instead.
Another general tip is to avoid holding down the trigger for full auto. Full auto LMG accuracy has been significantly reduced in recent updates. Run-and-gun makes it almost impossible to hit anything as desired. Take it slow, find good cover, and use slow burst fire. Your accuracy increases significantly with light taps, and doing rapid taps allows you to fire faster than you'd imagine.
Let’s start with: Get a kill streak of 5 with the MG3 SA. This one is tough. Due to the rate at which the MG3 om-nom-noms bullets, you probably will have to reload at some point before getting 5 kills. LMGs have notoriously long reload animations, which leaves you wide open to be killed. If you are Defending, run to an out-of-the-way corner, building, bunker, or anything out of the beaten path. If Attacking, depending on how far in you are, you can either head for cover in the enemy base, or fall back to your base. Although you can't fire your primary while reloading, keep your wits about you and be ready. If an enemy shows up, do not hesitate to pull out your pistol and defend your life. It won’t count towards your MG3 kill streak, but it lets you keep it going by prolonging your life. Just make sure you remember to switch back to your MG3 and finish reloading it. Also, although I highly endorse that the Medic is supposed to be a support player and make sure his teammates are healthy/revived, do not endanger your life to save someone while going for this specific award. Chances are, both of you will die after you revive him, and your streak will have been lost for nothing. Play it safe until you achieve those much desired 5 kills in a row. Avoid enemy vehicles at all costs.
Number two again: Get 8 kills in a round with the MG3 SA. Without the pressure of staying alive, this one should be much easier. The Marksman LMG perk should help you hit your targets more often, leading to more consistent kills. Stick to the sidelines, which is where most vulnerable infantry slip by as vehicles and aggressive foes take the frontline paths.
And finally: Reach 100 kills total with the MG3 SA. The MG3 is a beast of a gun; to those not familiar with its nuances, fire rate, and spread, it is wild and uncontrollable. To those who familiarize themselves with it, they turn their LMG into a thundering storm of terror onto their foes. In short, I think the MG3 SA kills will be either really annoying for you to complete, or really easy and enjoyable.
[7/10/10 Edit]: Removed stat screen glitch note (fixed in recent update), added bit discouraging rapid fire, fixed a few spelling errors.