I was wondering if it's correct to connect the washer drain and the bathroom drain all together.
I connected the washer's drain, the sink's drain and the bathroom's drain all together in the same line. When I started to use my washing machine, I noticed that when it was draining the water the water went to the sink and bathroom.:eek: What should I do? Or what did I do wrong?:confused:

It is possible to connect them all but you need to have a drain line sized so it can handle several fixtures at once. The minimum was a 2" pipe for many years but the 2009 code will start to require a 3" pipe because of problems like yours. I addition to being undersized there is also the chance that you have some blockage. I'd rod that drain line to be sure but I'd guess you washer is just overpowering the old drain which was likely 1 1/2" at best.