y=x-3a与y=-x+a-1
x-3a=-x+a-1
2x=4a-1
x=(4a-1)/2
y=x-3a=(4a-1)/2-3a=(-2a-1)/2
交与第三象限,则：
(4a-1)/2＜0
a＜1/4
(-2a-1)/2＜0
a＞-1/2
∴-1/2＜a＜1/4