Proposition: Prop. 11.05: Three Intersecting Lines Perpendicular to Another Line are in One Plane

(Proposition 5 from Book 11 of Euclid's “Elements”)

If a straight line is set up at right angles to three straight lines cutting one another, at the common point of section, then the three straight lines are in one plane.

For let some straight line $AB$ have been set up at right angles to three straight lines $BC$, $BD$, and $BE$, at the (common) point of section $B$.
I say that $BC$, $BD$, and $BE$ are in one plane.