There is no GPS as there is a thick layer of water on top of them. The signals from GPS on the surface are already very weak, they don’t penetrate even a few meters down in the water. Same problem with radio, you need some very special radio at specific frequencies to communicate that deep, a regular cell phone signal can’t go very far on land (relatively), water is a great absorber of radio waves.
If they weren’t tethered and have no special purpose communication device, they are effectively lost, 3km deep even if you find the relatively small capsule and they somehow haven’t drifted off for hundreds of miles, there are currently very few things you can do that could bring them up that wouldn’t instantly kill the occupants. The CIA (actually Howard Hughes, the Elon Musk of the era) once custom built an entire ship to recover a single Soviet sub that was at roughly the same depth, they knew exactly where it was located and it still took over a month once they got on site and they still didn’t recover everything (exact details remain classified, but if I recall correctly, the majority of sections of the sub broke off and sank again in the process)
This is a salvage mission, nothing more, nothing less, there was no backup plan for these people, no dedicated rescue sub, not even a locator beacon, maybe if they are able to make some noise then a sonar can pick it up.
If not dead yet, they will be dead soon, at best we can put a camera down and watch them die. If they had the ability to surface, they would’ve done so by now and been able to call for help. If they do somehow have the ability to attach a crane or something, even that would be very complicated, you can’t descend a human, even in a diving bell, the pressure is too high, you’d need special constructed diving bell/sub with robot arms to get there as the deepest scuba dive ever recorded was only 300m, not 3000m, get down there, attach them and then dislodge and hoist the thing up without compromising the structure. Perhaps possible, but not in whatever time they have left.