I would to build an adjustable 3-phase system operating between 45 Hz and 65 Hz, you are essentially designing a low-frequency variable frequency drive (VFD) signal generator rather than a standard entertainment audio amplifier.
‎Because precision phase shifting is difficult to maintain using analog resistors and capacitors across an adjustable frequency range, the most reliable approach is digital generation using a microcontroller, followed by clean analog amplification.

I need help on where to start.

I want to set up a test station for synchronisation and testing of DEIF controllers.

Any assistance will be highly appreciated.

 

Before designing a generator from scratch, check what the controller actually measures: DEIF genset/sync controllers expect VT-level AC voltage inputs (typically from around 100 V up to 690 V nominal, so check your model's datasheet) and 1 A/5 A CT inputs.
A microcontroller plus audio amplifier therefore needs proper step-up and isolation, and for synchronising you need two independent three-phase voltage sets (generator side and busbar side) with adjustable frequency, voltage and phase angle between them. Otherwise the controller never sees slip respond and will just time out on sync. Finally, if you mainly want to test logic and sequences, check whether your controller and utility software support DEIF's emulation mode before buying hardware.