Will installing a 2inch fitting in a 3 inch water line decrease water pressure?
Reducing the pipe diameter from 3" to 2" will not decrease the static pressure in the down stream pipes if all valves are closed and no water is flowing.

The restriction to 2" will cause a decrease in flow rate to the down stream piping assuming that the head pressure in the 3" line in front of the meter stays the same as with the original 3" meter. 

If your water usage at any given time is quite a bit less than the flow rate available through the 2" restriction then you will not notice a great decrease in delivered pressure. However if the demand flow rate approaches the capacity of the 2" restriction you will start to see a pressure drop off at the outlets due to the head pressure trying to push more water through the meter than can flow due to the smaller 2" size. 

My recommendation - If your pump setup is configured for multi-family service with 3" piping then go with the 3" meter.