To play seraph's shield you need to complete the seasonal story up to step 23. The normal version shouldn't be a problem with 3 people.
#Revision Zero
outer karma
For the catalysts you need to do legend which can be difficult. There's a lot of enemies in the boss room and the bosses are quite tanky, especially with 3 people and there's limited revives
But it never hurts to try
willow lynx
It’s not too difficult with a competent team
I’d compare the difficulty to something like prophecy, where soloing it is hard but doable and with a team of 3 you need competency but not much more
The gun is pretty explicitly a PvP gun, though, and by that I mean it’s one of the least satisfying and worst feeling guns in PvE
It’s like using a blue pulse, that after getting 30 precision hits allows you to fire a weaker arbalest shot