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: 
The wording on the bonus is really silly (same goes for the Nemesis ribbon). Once you get 5 kills, you're considered to be a on Combat Efficiency streak. Your 6th kill gives you a 1 streak bonus, 7th kill gives you 2 streak bonus and 8th gives you a 3 streak bonus and hence the ribbon.

It is possible to get multiple ribbons in the one life. Once you have got the first one, (I think) you will get the next one for every 3 subsequent kills in that life.

So if you go on a 23 kill streak you are going to 6 get of these ribbons (8 + 3 + 3 + 3 + 3 + 3)
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)