In reverse order:
3. It depends on your municipal water quality. Call the water company and ask for a copy of your last "consumer confidence report". It should have a hardness measurement. You'll have to look up what is considered "hard" but up over 50 and you start pushing it.
2. In general, weekly. Use half strength weekly (every watering for some). You could just go regular strength about once per month but I like making it always available. Some need a break from fertilizing so research your specific plants.
1. Avoid nitrogen as urea. Just look at the label. You want available N so the plant can uptake it immediately. MSU is perfect. First Ray's (OB member
) has it. It comes in different formulas for different water types. His site has lots of info too, you may get a better answer to #3.