Wash machine, sink all drains are backing up into bathtub.

You check the main drain past this point to the sewer line, Normally a clog of some sort, I would run a "snake" or "router" though the line to see if I could clean out any blockage.

Hi clogged and Welcome to The Plumbing Page. At AskMeHelpDesk.com.

You have a blockage downstream from the tub. The solution would b e to snake the drain line from the tub overflow. Let me show you how,
Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. The way you were going in through the drain it ran you square into a tee that you can't get around. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. After you snake pour a few pans of boiling water down the drain. This will loosen the grease and flush the mess out into the main. Hope this helps and thank you for rating my reply. TOM