Catalyzes the hydrolysis of 3-guanidinopropanoate to beta-alanine and urea. Possesses low activity against 4-guanidinobutanoate. Has no activity against arginine and agmatine. 3-guanidinopropanoate + H2O = beta-alanine + urea Binds 2 manganese ions per subunit. kcat is 106 sec(-1) with 3-guanidinopropanoate as substrate. kcat is 57.6 sec(-1) for 4-guanidinobutanoate as substrate. Homohexamer. Belongs to the arginase family. Agmatinase subfamily.