Exogenous shocks multiple values

Hi all,

I am trying to have multiple shocks in dynare and I follow Shocks on exogenous variables (Dynare Reference Manual)

var tfp = 1;
stoch_simul(order = 1,irf = 20);
gives IRF and no error message while

var tfp;
periods 1:3;
values 1.5 0.8 0.7;
gives an error: All endogenous are constant or non stationary, not displaying correlations and auto-correlations.

How can I add multiple shocks that tfp could take for the next three periods?