A private, for-profit college that offered training for medical, dental, and other health-care careers has been forced to close its three campuses, all in Northern Virginia, according to an announcement on its Web site. ACT College had been in rough financial shape for some time, appearing last year on the U.S. Department of Education’s list of institutions with low scores for financial responsibility. Its student-loan default rate in the 2009 fiscal year was 19.5 percent, according to the department’s statistics. The Associated Press reported that one instructor said he hadn’t been paid in a month. The college did not explain why it had closed, but said it would try to provide information to help students finish their studies.