lcarney
If it erupted with that much force, I would look to the sewer system. Have you contacted the local public works dept to see if they have any ideas?
I have heard of toilet ‘fountains’ being created when the DPW was using a high pressure jet to clean the sewer, and the home had inadequate venting.
If you are on the 3rd floor, it seems unlikely that a random backup would create that much hydraulic force.
Good luck! 
