The first one is a sigil (magical symbol) for Saturn, I've never seen other than on that album but I don't mess around in that sort of magic (hermetic stuff, apparently).
The second and third are actually variations on the same thing, called triquetra. The first one is more Celtic in flavor but all symbols containing threes were considered sacred, representing maiden/mother/crone, youth/father/sage, husband/wife/child, etc.
The final one is not a "classical" symbol, it's made up. I heard that it supposed to be representative of the feather of Maat that was believed by the ancient Egyptians that was used to weigh against your soul.