What does "in" mean in ancient architecture? "Three in, three out."

In and between mean the same thing as warp and weft. Between means the space between horizontal columns, and in means the space between vertical columns. The ancients used several rooms and several entries to indicate what size the house was. It's the same as saying 3 bedroom now.

The space between two columns is called "room". It is a more important architectural format in ancient architecture