A note on geometric 3-hypergraphs

In this note, we prove several Tur\'an-type results on geometric hypergraphs. The two main theorems are 1) Every $n$-vertex geometric 3-hypergraph in 2-space with no three strongly crossing edges has at most $O(n^2)$ edges, 2) Every $n$-vertex geometric 3-hypergraph in 3-space with no two disjoint edges has at most $O(n^2)$ edges. These results support two conjectures that were raised by Dey and Pach, and by Akiyama and Alon.