The 2-3 million figure is based on an _estimate_ by http://www.mediaradar.org/ and http://www.acfc.org/, which are not neutral sources (though this does not mean that they are wrong). The one clear piece of evidence cited is ~38,000 orders for Pennsylvania, which comprises 4% of the US population.

Edit: My initial comment understated the issue. Media Radar states that women are twice as likely to engage in unidirectional (unreciprocated) violence towards their partners are men. While domestic violence against men is real and subject to a lot of shame and silence, this estimate seems insane: I have previously seen figures like 10% of domestic violence stemming from women.