if you have a plot of land, it cannot be a developer :)
if you read a few posts here for just 5 minutes, you would have already read this statement at least 10 times and known that you can roughly calculate from a flat rate of 1500€/sqm + 40,000€ ancillary construction costs for a single-family house (+ all extras).
now it depends on what kind of "bulk discount" you get for the 3-family house and to what extent certain items are needed once or three times? 1x house connections? saves a lot. 1 big heating system instead of 3 small ones? saves a lot. etc... depending on how the 3 families are divided, you can either save a lot or nothing except a few fewer bricks on the outside (because a wall is shared by all).