I registered my pump account in pump2status.net (run by e14n, libre software) and then I have an automatic bridge to GNU Social (when my note is <=140 chars). In my GNU Social settings I have a TwitterBridge to my Twitter account. Like this, when I post in pump a note <= 140 chars, it get posted in the three places in some minutes.

Other people have setup the pump-twitter bridge in pump2tweet.com (rub by e14n, libre software).
That bridge allows notes >= 140 chars (they get truncated and the bridge adds a link to the original note in pump.io).