Is the smell coming from the sink drains or the shower drains? All drains in all bathrooms smell? How long has this been happening? Has your plumbing changed at all since the problem started?

You can find several older threads by using the search function. "bathroom odor"

Since you say it's all three showers, I suspect the drains. Pour a little bit of bleach down all the drains and see what happens over the next couple days.

A couple other methods to test:
If these are tub/showers, pour water down the drain and see if the smell is coming from the overflow. Maybe there's gunk in there.

If shower drains only, pour water down drain and see if smell comes out of the drain. If so, remove the strainer, then scrub the the walls of the drainpipe just underneath.

Next, how's the grout and caulk? Do you happen to have greenboard behind the tile? If so, any water that gets behind can cause a bad odor.

BTW, periodically you might also pour a little bit of bleach down the sink drains, too. Often that can be source of bad odor.