This was a paperback, and I was born in 1974 so I would have read this sometime between, day, 1980 and 1988. Probably suited for ages 6-12.
There were beautiful spidery-looking line drawing illustrations – almost like Edward Gorey’s but a bit more complex and not so cartoony.
There were witch sisters, Agatha and Hecate. They were not good witches. There was a misguided/bad man who worked for the witches named Oswald. The main characters were (maybe) siblings and the other main character was a girl they met who knew her way around the magical world where the witches were, and the lot of them got out of troublesome situations by stomping three times on a manhole cover. when they did that, they would instantly be whisked out of the place and land somewhere else. They used this at least once to escape the witches and Oswald.
This is definitely The Mystical Beast, by Allison Farthing, 1976
I think you want The Mystical Beast, by Alison Farthing.
Yes!! This is solved!! Bless you all. My daughter is now reading this book. The illustrations are even better than I had remembered. I will read it when she is done. Wonder if this woman ever illustrated anything else?? Her drawings are like Goya for kids.