How to make 3 parallel beam elements to work together in abaqus?

You would need to couple the nodes of two beams to the nodes of a master beam (likely the center one). That said, I'm not entirely sure this is a completely valid way to use beam elements. I think you'd be better off using solid elements or shells for this. That'll ensure you capture the transverse effects correctly.

sketch all three components seperately,Use section assignment to assign the scetion, and then assemble it.

or use shell elements , just sketch an I and then extrude it and then assign respective sections

http://imechanica.org/files/1_0.PNG

http://imechanica.org/node/18291