I rent my house and I board horses and we run our own family farm:) Somethings I pay for, some things the landlord pays for. Whatever I pay for, comes with me if I move! We may very well end up buying this place in the future too.
Where are you located ? If it snows you would need some sort of shelter/wind break , same if rains a lot. Is the 3 acres fenced at all ? Fencing is expensive even for T post and hot wire. Also are there stray dogs etc, that you would want to keep out, so the horse would not be chased ? Dogs do get out, and it is 'natural ' for them to chase. You need to consider fence cost, riding areas, feed for winter , can you use the pastures for grazing ? You should check with the neighbors first about access for riding. Then if you need a rain/snow cover then you also need to figure that cost. Also,, you have manure removal, fly control, and it is hard to find a good horse sitter.