I don't get it. Each of those calibers (5.56x45, 7.62x39 and 7.62x51) will require a different magazine and a different barrel to function. Are military units going to carry all of those? What will the unit armorers concentrate on? The 7.62x39 isn't even a NATO standard round. Supply is going to be a PITA and for what?
From the article linked in post 15:
Initial SOF requirements included two basic versions of SCAR system - the SCAR Light (SCAR-L), available in 5.56mm NATO, and the SCAR heavy (SCAR-H), which should be initially available in significantly more powerful 7.62x51 NATO chambering, and should be easily adaptable in the field to other chamberings. These other chamberings initially include the well-spread 7.62x39 M43 ammunition of the Soviet / Russian origins, and probably some others (like the proposed 6.8x43 Remington SPC cartridge, especially developed for US Special Forces). The key idea of SCAR rifle system is that it will provide the Special Forces operators with wide variety of options, from short-barreled 5.56mm SCAR-L CQC variation, tailored for urban close combat, and up to long range 7.62x51 SCAR-H Sniper variant, as well as 7.62x39 SCAR-H, which will accept "battlefield pickup" AK-47/AKM magazines with 7.62 M43 ammunition, available during the operations behind the enemy lines.
The 7.62 x 39mm is an East block round. It is used in the AK-47 that just about all of the people we're opposing have at the moment. Would be nice to pick up and use their own ammo against them, eh?