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(1) There are two situations when three straight lines A, B and C intersect at one point. A point intersecting on plane a; Two intersecting lines define a plane, and the third intersection point may not be on plane A.

(2) Three straight lines A, B and C have three intersections with each other.

Because two intersecting straight lines define a plane, the third straight line and the other two straight lines both have an intersection point, and this intersection point is on this plane, then the third straight line is also on this plane.

To sum up: a plane or 0.