@ewenmcneill that's absolutely it. Three grounds to help keep the impedance of the two pairs controlled and isolated from each other.
It's extremely common on high speed connectors to weave signal pairs with grounds between them. I.e. go look up the pin out for PCIe slots. https://pinouts.ru/Slots/pci_express_pinout.shtml
@ewenmcneill there's a non-standard convention that vendors like Supermicro uses to optionally inject 5V on one of the three ground pins to power SATADOMs, which are tiny SSDs that hang right off the SATA connector on the motherboard.