Final answer:
Telkom's market structure before the arrival of the big three was a monopoly, where it was the sole provider of telecommunications services without direct competition.
Step-by-step explanation:
The market structure for Telkom before the emergence of the big three competitors can be classified as a monopoly. Before other major competitors entered the field, Telkom was the only significant provider of telecommunications services, which means it was the sole seller in the market with significant market power and no direct competition. This situation is very different from perfect competition, where many firms sell identical products and act as price takers. It is also distinct from monopolistic competition, where many firms compete with differentiated products, and oligopoly, where a market is dominated by a few large, profitable firms. Therefore, Telkom had a monopoly market structure.