No. The pre out LR is for using an exemplifier for your Left and Right front(main) speakers. The Y splitter in your case would go on one of the subwoofer pre outputs. So from that splitter, one cable goes to the speaker and the other goes to the third subwoofer. For calibration you’d probably turn the third sub off, then just set the level and phase(if necessary)manually afterwards. As mentioned, DT speakers are unusual, and sometimes a real PITA.
I suggested bypassing the powered section of them, but as TLS guy said, that won’t work.
An honest long term suggestion would be to get rid of the DT speakers and replace them with normal ones and just use good subwoofers. Until then, use a splitter to the third subwoofer.