Specifically methylates the pseudouridine at position 1915 (m3Psi1915) in 23S rRNA. Specific for fully assembled 70S ribosomes. pseudouridine(1915) in 23S rRNA + S-adenosyl-L-methionine = H(+) + N(3)-methylpseudouridine(1915) in 23S rRNA + S-adenosyl-L-homocysteine kcat is 5-6 min(-1) (PubMed:20817755). kcat is 2.5 min(-1) (PubMed:28428565). Homodimer. Belongs to the RNA methyltransferase RlmH family.