Trimolecular system (normal modes)

[kmp]

there are three molecules joined through three springs of equal length 'l' forming an equilateral triangle , if all three molecules are displaced by equal lengths, they start oscillating linearly. write the simulation for both, the original problem and the linear one.

 

[kmp]

actually i want to write the differential equations for the motion and solve it by using Runga-Kutta method.
please help!