If you connect both voice coils of the third woofer in series they will be 4 ohm same as the original svc woofer.
You can figure it out with these formulae. http://physics.bu.edu/py106/notes/Circuits.html
Connect each dvc woofers terminals in series so they present 8 ohms. Connect the dvc woofers in parallel so they present 4 ohms. Connect the the svc woofer in parallel with the other two. The amp will see 2 ohms.
If you connect both voice coils of the third woofer in series they will be 4 ohm same as the original svc woofer.
You can figure it out with these formulae. http://physics.bu.edu/py106/notes/Circuits.html