Final answer:
The question seems to be related to the calculation of an internal force in a physics context, but it lacks sufficient detail to provide a definitive answer.
Step-by-step explanation:
The student's question involves calculating the internal force between points C and D, given three forces F1, F2, and F3. Without additional context, it is challenging to determine the exact nature of these forces and how they interact between points C and D. Normally, the computation of internal forces would require information on how these forces are applied (their directions and points of application), as well as the structure's geometry on which they are acting if it is a question related to mechanics of materials or statics.
Since there are different principles that could apply (such as equilibrium of forces for static situations or dynamics if the body is in motion), more information is required to provide an accurate answer. In the context of net force for a static problem, one would typically sum all forces taking into account their direction and apply equilibrium equations to solve for unknown forces. Alternatively, in electric force problems, a Coulomb's Law calculation might be used if the forces are due to electric charges. Without clarification, the question cannot be answered accurately.