I would go with what JonathanB said. I am not an ATCO as of yet, so I am ready to be corrected, but having the C172 first is probably not a good idea. Even thought it *might* give you the shortest space it is not the shortest time (actually the speed restrictions on the jets would be around their stall speed so, as JonathanB said, you would probably need larger space between them).
In fact, based on my understanding... here are some rough estimates of the Approach/Landing speeds of the three aircraft:
C172 = 110KT/65KT
737 = 250KT/140KT
747 = 250KT/160KT
All that without even taking into account number of passengers, connections, landing fees, .... there are quite a number of factors involved
.
... and yes... you are not supposed to know all this for your interview, as mentioned above, I believe it is mostly the reasoning process they are after.