Twitter allows you to update your Twitter timeline by sending an email to a secret address. So go to TwitterMail.com and sign-in with your Twitter account credentials.
In the Twittermail tab, find a button to setup your Twittermail account. Now simply, press the button “Set up your twittermail account now”.
In this Twitter setup page there is a unique email address which has been assigned to you to send the updates.
Now, when you wish to share a post in Twitter from your Google plus profile, just add that email address in the visibility box along with other circle(s) and share the post as usual.
To avoid having to type that email address every time, create a circle and add that secret Twittermail address to that circle. Voila!
Ditto except go to Facebook.com/mobile to get your facebook email address!
For chrome users, just install the extension for your browser and you automatically get fb and twitter buttons in g+ to share your post with!