Use 2 5V wires and connect them to slimline (i.e. Pin :- 9, 10). Leave the third 5V wire.
Use 2 GND wires and connect them to slimline (i.e. Pin :- 12, 13). Leave the other four GND wires.
The SATA to Slimline converters available in the market only seem to use one wire for 5V & one wire for GND. They bridge the two 5V & GND at the Slimline side.
Using one wire for each is fine but I would recommend you to go with the 2-wire approach.
