Ongoing argument and discussion in our company! At sizes below #1/0 AWG (70mm2) three and four core are the standard. Larger sizes are usually single core, especially for 5 kV and 15 kV circuits.
If the cable routing includes underground duct banks and/or conduits, a three core cable with earth wire is too large to fit in commonly available conduits or to be able to negotiate a conduit bend during pulling. Three core cable costs more than single conductors for the cable but single core usually takes more manhours to install since there are three or four reels to position for each pull.
Installation costs are proportional to weight of installed cable and devices. Single core reels are lighter and easier to handle. Pulling and handling three lighter single conductors may be less expensive than one heavy 3/c. (Our historical installation costs are not granular enough to determine the better method).
The counter argument is that the three core cable is difficult to handle in the cable tray or vault and the connection fitting costs are expensive. (No special cable glands for single core). If the project is in a high cost labor area, we may use more multi-core.
But in Africa, our affiliated company's project used all 3 and 4 core cables for 600V, 6.6 kV and 11 kV, including a lot of 1200 mm2, 3/c with armor. Labor was inexpensive and cable routing was designed to accommodate the wide bends.
In North America, there is a perception that all cables must be enclosed or protected in a conduit. Open wiring, as is common in the rest of the world, is frowned on by owners, inspectors and especially the electricians who are very good at bending and installing steel conduit. That requirement makes single core the design choice even though a multi-core routed in cable tray would be just as rugged and reliable.