If you want to be able to run all of those things at the same time, you will probably need more than one and a larger gas line coming into the house. Assuming a 2.5gallon per minute showerhead you need 1200 BTU/per hour per degree you need to raise the incoming water. In the winter around where I live, it starts out at about 34 degrees. So, say you want it to get to 114 to allow for a little cooling off on the way and to mix a little cold with it (this is still too cold for a dishwasher), you have to raise the temp 80 degrees, so 1200*80=96000btu for ONE shower going. Get my drift?
Trying to fill a tub or even a washing machine with one can be a pain. Some work better than others, but all of them will require more maintenance than a tank-type. Do a search for tankless here and you'll be able to read some comments both good and bad (mostly bad).