Amends federal gun-sentencing law to impose a new, mandatory minimum prison term for people who knowingly possess firearms in violation of 18 U.S.C. §922(g) and who have three or more prior "serious felony" convictions from separate occasions. Eligible offenders face a required prison sentence of 15 to 30 years, a fine, and no option for suspended sentence or probation. The bill defines what counts as a qualifying prior (offenses with a statutory maximum of at least 10 years and other listed crimes) and applies prospectively to crimes committed after the Act’s enactment date when the person already has the required priors; it also states the Act does not create a new right to challenge such sentences.