Hard to achieve (easily), since you want your Era 100s to function as both surround speakers for your Beam, and as ‘regular’ speakers for your turntable. 
 

Your likely best bet would be to add a Sonos Port (or any other Sonos device that has a separate line in) and sent the pre-amped (line level output) from the turntable to the the Sonos system. You could then group the Beam’s room, and set the surrounds to ‘Full’ to get the stereo output from them. Of course, the Beam, being in the same Sonos ‘room’ would play as well, but you could adjust the relative volume when streaming music versus when playing the TV input from the Beam.

Alternately, you could conceivably, although maintenance more difficultly, connect your turntable to the TV set, and play the music from the TV’s input on the Beam. 

There’s a whole area of this forum around turntable use, here. You may find others who have posted similar questions. 

If you just have 100s and aren't using them as surrounds, Sonos sells a line-in adaptor that will give you a 3.5 mm input jack. Plug the turntable into that and it should play fine.

If you have a line-out on your TV you could also Plug it in there but there will be a slight delay in the sound, putting lip sync off a bit.

Get the Sonos adapter, other  brands almost always don't work.

brennon vb1 £79