If your turntable already has an amp and speakers, you will need a Sonos Connect. You will also need a phono pre-amp. Some turntables have it built-in, for those that do not, you can buy an external model. Connect the output of the turntable/pre-amp to the line-in on the Connect. Connect an output on the Connect to the amplifiers input (analog, coax, optical) of your choice. Note: If the turntable is currently connected to a phono input on the amp, you cannot use this because the output of the phono pre-amp is no longer equalized for a phono input.
Now in the Settings for the Connect room name, choose Line-In Settings and turn Autoplay to on. Choose the Connect room name for the Autoplay room. This will start the music playing whenever you drop the needle. You can then group the Sonos 3 with the Connect. As a bonus, you can also play any other Sonos sources over the amp and speakers.