Quartiles when all numbers are same I understand quartiles are to be used in large data sets. But for pedagogical purposes, What would be the Quartile1,Median and Quartile3 for a set consisting of same numbers? What would be the quartiles of 7 times 3?

For corner cases like this, you need to consult your definition. All the definitions out there agree on large continuous data sets, but they differ in detail when individual observations matter. Wikipedia gives three methods for computing the quartiles. If your set is seven samples, each with a value 3, the only thing that makes sense to me is to have all three quartiles be 3 as well. All three Wikipedia approaches agree in this case.