I'm usually quite critical of trademark law as I generally think giving exclusive rights to a single company over common English words is wrong.
I do, however, think in this instance Twitter actually have a point - "Tweet" has long been associated with its service, the word itself is a variant of the name "Twitter", and all the early references I can find explicitly refer to Twitter itself.
All three of the other pending marks are based on Twitter's own earlier (unregistered) use of 'Tweet'. So Twitter might be able to argue those should be rejected based on their earlier common-law usage.
Or, they could buy off those three applications.
Or, a combination of both. ("Do you folks want to make us your enemy, and take a chance that your registration will be rejected based on our earlier creation of the 'Tweet' term, or take this bag of money/stock/thanks? Your choice, tiny companies dependent on our ecosystem!")
The verdict is still out on this (with "first use" being Twitter's ace in the hole), but the article is an informative update on this situation.