I kept thinking of a cross between Captain America and Khan, so I think it works fine for d20 Modern. But you should do more than just straight stat boosts.
Give them a few miscellaneous other bonuses and +3 would be perfect. For example, maybe they heal naturally (keep it a slow rate, say 1 HP per minute?). They should have Low-Light Vision. Maybe they have Fast Movement (+10'). If you drop the normal Human bonuses (Feat and skill points), you could stack a few more things on.
Let's see... using Soldarin's system (far better than anything I've seen from WotC), you could do:
SUPER-SOLDIERS
+4 STR: 0.4
+4 DEX: 0.4
+4 CON: 0.4
+4 INT: 0.2
+4 WIS: 0.2
+4 CHA: 0.2
Speed +10': 0.2
Low-Light Vision: 0.0 (lack of this is -0.2)
Fast Healing: normally 0.5, but since I'm making it be so slow let's bump down to 0.2.
Natual Armor +2: 0.4
So far we're at a total of 2.6, which'd round down to 2, so let's add a few random skill boosts to put us over 3.0. For example, +2 to Listen, Spot, Climb, Swim, Tumble, and Balance. Now we're at 3.2.
There, an ECL +3 race.
EDIT: Forgot to say, Soldarin's system seems to work even better for d20Modern in some ways. Without magic-wielding classes, the physical stats can be much more useful than the mental ones, so having INT, WIS, and CHA only cost half as much isn't a bad thing.