asymptotic approximation of a third-order differential equation

Question

I am investigating a 3rd order differential equation of the form $$v'''(t) + X(t)v''(t) + Y(t)v'(t) + Z(t)v(t) +U(t)=0$$ It is known that the coefficients $X(t),Y(t), Z(t), U(t)$ exponentially tend to some constants. Please tell me the literature that will allow me to consider the asymptotic approximation of my equation in the form of a linear differential equation with constant coefficients.