Three bays and three entrances' means that all three of the front bays serve as entrances (Japanese Zen Buddhist temples usually have a Sammon gate with 'five bays and three entrances').
「三間三戸」は門の正面柱間が3間で、3間すべてが通路になっているものを指す（日本の禅宗寺院の三門は一般的には「五間三戸」である）。