Your code had some problems.
You were not declaring v as a function.
Your definition of eqn3 involved multiplying eqn2 by a value, but eqn2 is an equality and multiplying it by a value as part of an expression is dubious.
I had to guess what you might possibly mean by multiplying by eqn2 there. I probably guessed wrong, so review your system.
syms x(t) f(t) x3(t) g(t) v(t)
eqn2 = diff(v) == -3647.333890*x-99.48571587*v+3947.845868*x3+3947.845868*g+0.0002385211688*f;
eqn3 = diff(x3) == -0.004499581734*rhs(eqn2)-0.71466948*v-28.3599*x+28.3599*g;
eqns = [eqn1; eqn2; eqn3]
vars = [f(t), g(t), v(t), x(t), x3(t)]
[V, S] = odeToVectorField(eqns)