There should be a fitting into the manifold to supply vaccuum for the wipers.
If none of the fittings in question appear to go into the intake portion, just blow some compressed air or water from your garden hose into one of the fittings and see if it comes out of just one or both of the fittings. I doubt it matters which ones you use (if all 3 can be used).
Since your the one with the intake, posting a picture might help.
