How can I represent a schematic diagram of circuit for three D-cells placed in a battery pack to power a circuit containing three light bulbs in parallel?
Even though it is not given in the problem, all cells should be connected in series.
It is a good practice to connect a switch in series in the circuit always.
It is a safety feature.
Your circuit looks like this:
Notice that a switch has been connected in series with positive side of wiring with each bulb.
It is also not given if you are using Normal cells of 1.5 V each type
or Rechargeable cells of 1.25 V each. If you know then appropriately mention the voltage of the battery pack.
It is recommended not to attempt mix and match of different types of cells. Use all three D cells of the same kind.