Answer: Not very. Details here.
The three month forecasts by NOAA and others are short-term climate (i.e., “average” weather such as “warmer than average” or “wetter than average”) forecasts.
Some seasons, the forecasts work out quite well. Meteorologists know that the atmosphere is more “forecastable” at sometimes than others, but we still do not know why. We have made tremendous progress over the last ten years in forecasting out to three days and there is a fair amount of skill at five days. Three months is another matter entirely.
And, there is zero ability to forecast weather or climate three or thirty years into the future.