Catalysis of the reaction: R1-L-rhamnose-(1->3)-alpha-L-rhamnose-R2 + H2O = R1-L-rhamnose + L-rhamnose-R2. This reaction is the hydrolysis of an alpha-(1->3) linkage between two rhamnose residues in a polysaccharide chain.
endo-(1->3)-alpha-L-rhamnosidase activity