What is this 3 core cable?

Question

Computer networks

When conducting cable Internet into the house, I was led such a cable .. without insulation, and after an hour of searching on the network, I didn’t find anything like that to explain its belonging, I thought it was telephone, but the latter seem to have at least 4 veins. There is such a 3-core cable, without insulation. Does anyone know what it's called and how it's used?

Answer 1

Answer 2

I already thought that somewhere the Soviet three-vein still lives ...
You have optics.