A three point mounting system is just what the name implies, the prime mover and generator on mounted on a single rigid frame, and the frame to foundation interface is thru three sets of vibration isolators, usually with the side having two isolators groups being the side seeing torque reaction.
You this arrangement commonly on oil rig and large EPG applications. It is the "standard" mounting arrangment for some CAT medium speed units, and some Solar turbine packages, and I have seen competative products mounted in that fashion as well.
I have always been told the theory behind this type of mounting is to provide better vibration isolation and to reduce frame deflection during transient conditions. The following comes from the CAT EPG Application and Installation Guide.
"Three-point mounting rails are used if the rails are to be secured to a base or foundation that may subject the mounting rails to distorting forces. Suspending the power unit on three points isolates the unit from deflection of the substructure, thus maintaining proper relationship and
alignment of all equipment and preventing distortion of the engine block. More than three mounting points can cause base distortion as shown in Figure 6. The three point
mounting rails can only be used forclose-coupled driven units where the rails are extended for the mounting of these units. Objectionable vibration can occur if the power unit is not mounted on well-supported structures or is not
anchored securely. In addition to the three-point mounting, vibration isolators may be required to isolate objectionable vibrations."
Sorry, figure 6 didn't copy into this page.
Hope that helps.
