A rifle-wielding madman who allegedly fired indiscriminately at cars on a busy Massachusetts road had been released from a psychiatric hospital just three days earlier – and had boasted about getting away with murders, according to authorities  Tyler Brown, 46, was released from McLean hospital in Belmont last Friday, before then allegedly injuring two men during Monday’s crazed rampage in Cambridge, according to a police report seen by The Post. Brown, who has a criminal history spanning two decades — including trying to kill a cop — had “previously been diagnosed with post-traumatic stress disorder, anxiety, and depression,” Massachusetts State Police Trooper Christopher Hardy said in his report. Tyler Brown was identified as the gunman behind Monday’s shooting in Cambridge, Massachusetts. Boston Regional Intelligence Center The crack cocaine user expressed “suicidal intentions” before he unleashed a barrage of bullets while walking down Memorial Drive armed with an assault rifle, the documents said.…