Why nitrogen and hydrogen has formula of NH3?

Nitrogen element is located at V (five) group of periodic table, this means it has 5 outermost electrons. So, its atom needs 8 - 5 = 3 electrons in order to be at steady state of noble gas, this mean its atom needs 3 hydrogen atoms. Thus, NH3 is more popular than NH2 and NH4 (they aren't both steady)