If you feel like you need more than three competitors to make sure that you get a better sample of the anchor text being used for your keyword, then that isn't a problem. You merely create the initial keyword with 3 competitors then you can add more as you go. Here is how you do it...
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When you initially add a keyword to a campaign you are presented with a form that looks like this… 
As you can see for the initial campaign create phase we require 3 competitors to make sure that we get enough data to come up with an ideal distribution. If you would like to add even more competitors to your calculation, you can add them by browsing to your keyword overview page, and clicking on the plus button next to “Competitor”. 
Now you will be presented with a similar form, where you can add a new competitor. 
