How to create a Frequency Spectrum plot from nonlinear equations

given three nonlinear equations

dx1/dt = 3(x1+x2)

dx2/dt = x1 - (1/6)x2

dx3/dt = (1/7) + (x1 -3.25)*x3

and their correspoding initial condtion x1(0) = 0, x2(0) = 4 and x3(0) = 0, I able to obtain t,x from using ODE 45, but after that I am quiet lost how to create a frequency spectrum from using the three nonlinear equations. please help! Seems like all the output from equation are non-unifrom therefore, interpolation has to be invovled but not sure how to do it. Please help with through explanations!