Replacing a wall outlet with three black and three white wires with a smart outlet with one white and one black wire
If your existing blacks were all on 1 side of the receptacle on the brass screw and the whites on the other side silver screw than yes you can tie them together.
Why 3 cables , possibly 1 feeder the one that supplies power , one going to the next receptacle and one feeding a light switch. This would be quite common.
I would say possibly every house I have wired has at least one or 2 of these configurations.