2002 Oldsmobile Alero misfire

Engine Mechanical problem
2002 Oldsmobile Alero 6 cyl Automatic 140880 miles

last week the check eng light came on and it was a misfire in cyl 2 so we put brand new spark plugs in and than again today check engin light came on and said misfire in cly 3 it also bogs out if u go over 50mile and hour and the check engine light flash intill u get under 50 what could this be