Are you thinking of a power strip?
hi.what do you call something that you plug into a socket(outlet) and you get 3 outlets by means of that?
I think "power strip" may be AmE. Over here, we'd call that a 'trailing socket'. But as the original question mentioned three specifically, maybe it's referring just to an adapter (usually a little white box that plugs directly into a socket and has no visible wires).
I'd call it a 'three-way adapter'.
"Three-way adapter" would be the sort that plugs into a socket without any cables trailing everywhere, but "adapter" is a very vague term, so you'd have to make sure the context made it clear what was meant. For example, an adapter may be the device you use to be able to plug a device with a French plug into a British socket; or to step down the voltage from 240V to 110V.
In the US you can buy a "triple outlet adapter" or a "multiple outlet adapter".