Euler Buckling

1. The problem statement, all variables and given/known data

I want to find the euler buckling for a open I beam( middle section like a box) that is subjected to three point bending with a point load on top.
I am not so interest in a solution but explanation on how this could be worked out using the euler equation for buckling.

2. Relevant equations
Pcr=(PI^2)EI/L im pretty sure this is for a column with pinned ends but don't know how to work out one for an i beam in 3 point bending

3. The attempt at a solution
Don't want a solution can put numbers in myself

 PhysOrg.com science news on PhysOrg.com >> Front-row seats to climate change>> Attacking MRSA with metals from antibacterial clays>> New formula invented for microscope viewing, substitutes for federally controlled drug

Recognitions:
Gold Member
Homework Help
$$f_a/F_a + f_b/F_b <= 1.0$$ , where
$$f_a$$ = actual design axial compressive stress
$$F_a$$ = Allowable axial compressive stress as if there were no bending (this is generally the Euler buckling stress pi^2EI/KL^2(A) for K=1, with appropriate safety factors)
$$f_b$$ = actual design compressive bending stress
$$F_b$$ = Allowable compressive bending stress as if there were no axial load (this allowable stress must take into account lateral torsional buckling of the flange due to the compressive bending stresses, with appropriate safety factors).