snRNP U1,U2,U4,U5,U6 are present in the spliceosome but there is no mention of U3.
U3 RNA with its proteins forming a complex is localized at nucleolar. The box C'/D motif in U3 snRNA is responsible for this localization. Therefore U3 snRNA does not have a chance to go to mRNA transcription active sites. Instead, U3 snRNP play important roles in ribosomal-RNA processing. Because of necleolar localization, U3 snRNA is called U3 snoRNA alternatively.