CoD4 achievement HALP!!!

I can't get the 3 of a kind achievement on CoD4 and its beginning to annoy me (gonna prevent my 1000/1000 sooner or later). Now, I've stabbed 3 enemies in a row, but the achievment never unlocks. Does it have to be within a certain time limit or something?

thanks guys.

I cannot offer you "halp", but I can offer help.

The achievement has to be done within 10 seconds or so (not sure if it's ever been confirmed). I just played a level on Recruit, and got it in no time (where there's a high density of enemies).

1st mission...

Right in the beginning...

After you rope down... your team will open the door...

Sprint ahead of them... down the stairs...

Run down the hall... knife the guy... run in the room to the right...

Stab the guy on top... crouch... stab the second guy...

Do this all very fast... the achievement isn't for just 3 in a row... it is for 3 in a row quickly...