how i connect my three different company within 1 kilometer at possible chepest technique?

Answer 1

VPN's (virtual private network) is the cheapest way to connect remote computers without indluging them into the internet. However the actual trasmission will be over the internet but it will be private and no outside person can get into the information being passed.

Answer 2

1. You can have a virtual network
2. You can have a fibre cable physical connection (have done it in a company I worked earlier