Hi , I need help, shower is not draining, and when I flush the toilet the water is backing up to the shower, also when I use kitchen sink same thing, the water is backing up in the shower, I tried the strongest industrial drain opener but still nothing, please HELP I am so desperate!

Stop using chemicals for this type of clog. You are wasting money and are endandering anyone who opens the drain pipe. You need to auger the main drain line, either by rental or have a plumber to it. Hopefully it's grease or hair, not a broken line.