Can you control the VFD from mach3?

Yes, you can control the VFD using the a relay. You will need to connect the relay output (NO) to the forward terminal of the VFD and the gnd terminal. You will need to look at the diagram for your VFD to determine which gnd to use in the set of terminals that is associated with the forward terminal. The forward terminal may be called something else, but the manual will tell you (something like X1, or D1, etc.) You will also need to change a parameter in the VFD to use the external terminals as opposed to the control from the control panel.