I did not answer the second part of your questions. I have never heard of nor seen er 80sb3 or er80sd3, but ER80S-D2 welding wire is designed for the gas metal arc welding and gas tungsten arc welding
when moderately high strength and good sub-zero toughness is needed in the PWHT condition, and ER80SB-2 is for gas metal arc and gas tungsten arc welding of 1 ¼ chrome, ½ moly steels used in high temperature applications. Again if it's P91 material it should be ER90sB9.
Jim