Any of these will work Dave.
The 798cXi Si should plug onto the Matrix 27’s base and will be able to use the power cable, GPS Receiver and XHS-9-20-T DualBeam transducer. If used with only the DualBeam transducer the 798 will not have Si or Di sonar functions. I would set the Transducer Select menu to the DualBeam setting when using it this way.
If you use the AS-Si-DB-Y cable, the 2D (DualBeam) sonar will always come from the XHS-9-20-T transducer.
If you use the TS3 Transducer Switch you can select which transducer will be supplying the 2D sonar to your 798 unit. Again, I would set the Transducer Select menu to the DualBeam setting when connected to the XHS-9-20-T transducer.