I'm fairly new to horses, so all I can offer is what I've learned over the last year and a half of having them. First, no matter what you do, 3 acres isn't going to produce enough grass to keep them fed. You will have to buy hay and feed. We give ours a scoop of sweet feed every evening, which is also where we add our wormer and other medicines or supplements as needed.
Fencing is more important then clearing every tree. Remove all the small saplings, but leave the bigger trees. In my experience here in the south, the grass grows better where it gets some shade during the day. The more sun it gets, the dryer the soil becomes. The thickest grass that I have gets half a day of shade.
Horses are also picky on what grass they eat. I have some that is eaten down to the ground so all you can see is the color of the grass. They really like that grass. But ten yards over, what looks like the exact same grass is two inches tall.
To help keep the brush under control, I'm up to four goats now. I started with two and they did such a great job on blackberries, and other unwanted plants, that we're increasing our goat numbers.
When it rains, they all run to the barn. I was told that horses didn't mind the rain, but I've found that if they can get out of it, they will in a hurry. I have a pond and I have a water tank that is filled with city water. They love the city water a lot more then the pond.
I have two rows of electric white tape on T posts every 20 feet. They have all been zapped at least once, and never challenge it. They touch it with their nose, and you can see their tracks dig in and spin where it happens. I've only seen one get zapped, and she went straight up, spun around and took off running. Lesson learned. Nothing is cheaper then a hot wire fence, but you have to make sure it's kept clean from debris and weeds. I like the white ribbon because it's very easy to see.
Find a vet and farrier before you need them. You'll learn that everyone knows who the good ones are, and you'll also learn that there are a lot that others have used and wont use again.
Good luck.