S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the N(3) position of uridine 2634 (m3U2634) in 25S rRNA. S-adenosyl-L-methionine + uridine(2634) in 25S rRNA = H(+) + N(3)-methyluridine(2634) in 25S rRNA + S-adenosyl-L-homocysteine Present with 2960 molecules/cell in log phase SD medium. Belongs to the class I-like SAM-binding methyltransferase superfamily. BMT5 family.