(1)y=-x2+2x+3与x轴的两个交点为A、B,
0=-x2+2x+3,
解得:x1=-1,x2=3,
∴A(-1,0)B(3,0),
∵与y轴交于点C,
∴C(0,3);
(2)y=-x2+2x+3,
=-(x2-2x-3),
=-[(x2-2x+1)-4],
=-(x-1)2+4,
对称轴x=1,顶点(1,4);
(3)(-4,3)或(4,3)或(2,-3).
(1)y=-x2+2x+3与x轴的两个交点为A、B,
0=-x2+2x+3,
解得:x1=-1,x2=3,
∴A(-1,0)B(3,0),
∵与y轴交于点C,
∴C(0,3);
(2)y=-x2+2x+3,
=-(x2-2x-3),
=-[(x2-2x+1)-4],
=-(x-1)2+4,
对称轴x=1,顶点(1,4);
(3)(-4,3)或(4,3)或(2,-3).