Question: what is the chance of catching a legendary pokemon?
Answer: Legendary Pokémon can be caught after being defeated in Legendary Raids, but their Capture Chance is very slim. With a Base Capture Chance of 3%, Articuno, Zapdos and Moltres are a bit easier than Lugia (2% BCR). This page goes into details on how each ball/berry/throw type influences your chances.