For what you want, it seems to me that 10-15 acres should work out about right for you. Not sure why 3 Dexters vice 2... one pregnant and one lactating... Unless you wanted more overlap. Really if the land you buy has a 10 acre requirement for the 3 Dexters, then you break that into 3-4 pastures and rotate the cows through each. That gives each a chance to recover between use. I hope you find the ideal property for what you want to do!