Back to economics.
For the "less expensive" system you need 1 VFD, six contactors mechanically and electrically interlocked, 4 CB disconnects, two sets of leads to each motor, and wiring six contactors. You need two sets of I/O for each motor (fault oon bypass, running on bypass) plus fault on VFD, and running on VFD. You do save on analog I/O, since you only need one set. You also need to do more programming in your PLC, and a lot more testing and de-bugging. (We just got done with a project in Spain that had existing constant speed, and converted only one to VFDs - believe me, the programming and testing are not inconsequential!)
For the other system you need three VFDs, three CB disconnects, and wiring for three motors and VFDs.
How much are you really going to save? What is the motor hp and supply voltage?