The bending moments diagram should be drawn per definition on the “tension side” of the beam. In Plaxis 3D2011 however it may be that the bending moments M2 and M3 are plotted on the wrong side of the beam and/or the sign is wrong. Note that the absolute value of the bending moments is correct. The displacements of the beam are correct as well.
Plaxis will solve this issue in the next version.
Example: simply supported beam
This issue is now fixed in PLAXIS 3D2012