To determine the maximum total network length of a Profibus-DP network when three segments are connected using repeaters, we need to understand the basic rules governing Profibus-DP network configurations.

Profibus-DP networks are often employed in industrial settings for high-speed data communication. At a maximum speed of 12 Mbps, the maximum allowable cable length per individual segment is typically 100 meters. When repeaters are used, additional segments can be added to the network, effectively extending the total reachable length. However, each segment added with a repeater can extend the network by only its own allowable maximum segment length.

In the given problem:

• The maximum length per segment at 12 Mbps is 100 meters.
• Three segments are connected using repeaters.

The maximum length of each segment remains 100 meters, and each repeater connects these segments without adding any additional length.

Therefore, the total maximum network length can be calculated as:

\(3 \text{ segments} \times 100 \text{ meters/segment} = 300 \text{ meters}\)

Therefore, the correct option is 300 meters, as this accounts for the maximum allowable length when each of the three individual segments is fully utilized.