You don't say what grade of SA-182 is being welded, such as F11, F22, etc. There are many grades of SA-182 ranging from low alloy steel, to high alloy steel to stainless steel. Since you said Cl. 3, it may be either F11 or F22. If so, you could weld it to SA106 Gr. B pipe with either E7018 or the matching filler for the fitting (E8018-B2 for F11, E9018-B3 for F22).