Why does my bath tub drain back up when I try to unclog my toilet... that now seems not to be clogged with anything.. the only thing that I see are bubbles... and when the bath room sink drain it causes the toilet to over flow?

Hello fgnola,
You have blockage past the connection point where Sink, Toilet and Tub connect into common drain. Sink is the highest fixture in the bathroom so when water from the sink is released the water column will push waste water into the toilet or shower drains. When you try to plunge toilet, you then push water into the tub.

I drew simple sketch to give you better visual...

Solution: You have to clean your sewer pipe. You can remove the toilet and insert large snake through closet ring (... that's the hole in the floor where toilet was seated ). You will feel resistance where snake hits the blockage. Go past that point and back several time to assure you get it all dislodged and pushed down the drain. Fill tub with water and release it to the drain. It will flush it to to the City sewer.

You can rent snake from Home Depot for few hours or call Roto Rooter.

Milo