Smallest network

Question

Is the smallest network 1, 2 or 3 computers ?

Answer 1

Speaking in terms of the actual network: TWO.
You could have a /30 network (subnet: 255.255.255.252) which would provide one network address, one broadcast address and two usable host IP addresses (for computers or other devices).
Such a small network is common with point to point connections between two office locations where two routers (one in each location) would each be assigned a host address.