building box beams

This is all of our wood, or most of it.

The floors are made of box beams.
The box beams are made of two 2x10's and one 2x4. The 2x4 is sandwhiched between the two 2x10's and is 3.5" shorter on the sides.
A fair amount of the work is just looking and cutting down the 2x10's.
While a seemingly simple tasks, keeping the beams square and true is not easy.