When drawing duct, Revit will place the elbows automatically, based on the settings within that type of duct. You edit the duct type, and select "Routing Preferences". you can then choose what type of elbow to use. You cannot select specific angles for the fittings, for that depends on the angle you draw the next duct from the first. the fitting will be automatically placed.
As for the 3-way fitting, You will have to load a family in from the Revit library ("Insert" Tab, "Load Family"). There are many different types of fittings, you just have to try some.