what order were they in? And what time of day did you take them?
If i remember right, first thing in the morning would have the highest concentration of the pregnancy hormone?
Did the two positives come with the first two tests, and the non-positive one was the last, or third one? If so, maybe there was a very decreased, or undetectable, amount of hormone in the third test? I'm sure these tests are very sensitive to the hormone , but, you never know.
If the first two ones were very faint positives, you may not have a lot of this hormone built up yet. The above example may be why the third didn't, or wasn't able to, materialize on the third test.. Are you testing really early?