you can not connect 3 wires to the one sma end for rather simple reason, each connection has it's own amplifier. Connecting them together will terminate or substantially worsen functionality. On the other hand, by using signal splitters, you can connect external antenna. But for improvement you gonna need something like this

still, extra sata controller, don't see it on TC. additionally it appears second SATA connector was provisioned. ifix it, aren't we here to fix?