Question: What constitutes a Streak Bonus? This is in relation to obtaining the Combat Efficiency Ribbon, which states that you have to obtain 3 Streak Bonuses in a round.

Answer: 
Combat Efficiency Ribbon 
500 XP Bonus. Get 3 Kill Streaks in one round. To get a streak, you need to kill 8 times in a row without dying yourself. Get 3 Kill Streaks of 8 to unlock this ribbon, try playing on infantry heavy maps to increase your chances early on.

Per <http://www.gamefront.com/battlefield-3-ribbons-guide/>
After 5 kills without dying, every 3rd thereafter is considered to extend the 'streak'.

Thus, to get three in a row, it would be 14 kills. (5 +3 +3 +3 = 14)