There is no simple answer to this question because so many factors come into play. Here are a few questions/topics that need to be considered:
1. Are you planning to have the forage from the pasture be considered as a part of the dietary intake?
2. What type and quality of forage and soil exist on those three acres?
3. What have you done or will you do to support soil/forage health, in terms of both fertilization/adding compost/etc., AND protection and maintenance of the grazing area by rotational grazing, keeping horses off during wet/muddy or recovery periods, etc.
4. Fencing, equine activity levels, whether they can be kept together or must be separated, age and use, amount of turnout time, whether there is a high traffic/sacrifice area for horses during pasture recovery time, etc.
As can see from the above topics, there are many aspects to your question that vary by location, management, growing season, and much more.
In addition to the articles and topics on www.extension.org/horses, you may find some useful information on pasture management and establishment at
http://pss.uvm.edu/vtcrops/.
