I'd appreciate advice on the best way to construct a 12-foot wood beam
that is strong, sag-resistant and easy to assemble and disassemble.

The plan is to construct the beam with two rows of 2x3 spruce or
hemlock lengths with their sides bolted together to make up a beam
that is 2-1/2 inches in the vertical dimension and 3" in the
horizontal.

The total load on the beam would be about 30lbs. and would be
distributed evenly along the full length of the beam.

I'm particularly interested in knowing how many lengths should be in
each row, how long each length should be and where they should be
positioned in relation to each other. No length can be longer than 8
feet. Any other information that would help me construct a strong and
sag-resistant assemble/disassemble beam of 2x3 lengths would be
welcome as well.

Thanks,
Darro