The flows are for 15 years, and therefore are from 4 – 18
So either take the 18 year annuity factor (which you would have to calculate yourself) and subtract the 3 year annuity factor.
or
Take the 15 year annuity factor and multiply by the ordinary 3 year factor, because the annuity starts 3 years late (i.e. at time 4 instead of time 1).