(Portland, OR)  --  Police are investigating three separate shootings and a stabbing in the Portland area.  The Portland Police Bureau says the four incidents all happened within a 12-hour period Saturday across the city.  The first incident happened around 12:30 p.m., when a woman was shot in the arm in an apartment complex parking lot in southeast Portland.  A drive-by shooting more than three hours later in northeast Portland didn't cause any injuries.  Later Saturday night, a 15-year-old boy was stabbed in the back near Southeast Alder Street and 174th Avenue and hospitalized for serious injuries.  The final incident happened roughly an hour later when someone opened fire on a vehicle carrying three people, nearly hitting a 6-year-old girl inside.