Question based on scaling property of dft
Some hints that should help you:

From the scaling property, if we stretch the time axis, we compress the frequency axis. 

It is also important to know the periodic property of the DFT: If you extend the frequency axis beyond N samples, the DFT will repeat. Consider a repeating DFT spectrum that extends out 3N, and consider that you now have 3N time samples. You stretched your time waveform from N to 3N, so what occupied 0 to N-1 in the DFT before will now be in the first third of the spectrum (roughly 0 to (N-1)/3). To understand what occurs in the rest you need to be aware of the periodic property of the DFT.

Process some simple DFT's with zero inserts on your own this will also be more obvious to you.