In a CV - I taught vs I was teaching vs I had been teaching

In the context of your CV, you are explaining things that you did. Simple past (your third example) is best, because you are not attempting to place your statement in a time context with another statement. Look at these:

I was teaching my students when the fire alarm rang and we had to leave the building.
I had been teaching for five years by the time I turned 25.
I taught academic oral language for five years.

In the first example, teaching is an activity that was ongoing when another event occurred. In the second example, teaching is an activity that occurred for a specific time period in the past that ended before another event occurred. (These two are somewhat interchangeable. For a good explanation of the difference, see here and here.)

In the last example, teaching is an activity that occurred during some five-year period in the past. This is most likely what you want. If you wanted to use one of the other tenses, you'd need to compare teaching to other past events, and those events wouldn't be relevant in your CV.

Now, one other possibility is the present perfect:

I have taught academic oral language.

This is fairly common if you are describing past experience that you have now: I have taught academic oral language, I have sold vacuum cleaners, etc. If you are describing the present state of your past experience, this is typically the tense used.

2 is not correct only because you said you already quit your job. That means you are not teaching any more.

"I had been teaching" implies that you are still teaching but you are not. Makes sense?