What is the different between 3phase 3 wire and 3 phase 4 wire energy meter?

It means the first one is 3 phase meter with 3 wires and the second one has a nuetral too. The former can be used for delta connected load and the latter for star connected load

In any installation,there is lighting or single phase load too. The utility usually install 4 wire kwh meter including neutral. Can you give an example where 3wire kwh meter is used.

The difference is star & Delta connection