S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the N(3) position of uridine 2843 (m3U2843) in 25S rRNA. S-adenosyl-L-methionine + uridine(2843) in 25S rRNA = H(+) + N(3)-methyluridine(2843) in 25S rRNA + S-adenosyl-L-homocysteine Localizes predominantly to the cytoplasm, with minor localizations in the nucleus (PubMed:24335083). Associates with pre-60S ribosomal particles (PubMed:24335083). Belongs to the class I-like SAM-binding methyltransferase superfamily.