My husband and I are in the planning stage of building and want to make it as simple as possible (for the humans)and enjoyable for the horses.
It is a small place (by southern standards) 6.5 acres w/only an additional 2.3 acres connected and available. Mostly level property w/a gentle slope on the west side. We have room for three separate pastures hopefully converging just behind the house and barn. Here is what I would like to do..

I would like to provide one large vessel for water that sits at the point where all three pastures meet. creating fencing that allows the horses to access the water no matter which pasture they are in.. (If the pastures were a pie and you equally divide them into thirds, in the very center of the pie sits the water vessel. if that helps explain it)

We are zoned E-MF (estate mini-farm) and are required to use a 4-rail wood fence anywhere the fence can be seen in the neighborhood. (we are a corner property, so that will be most of our fencing) So I was thinking we could somehow have a large tub - like a barrel type wood tub, positioned at the convergence of fence. Maybe only have the top 1 or 2 rails in place over the tub to stabilize the fence. We would have water line run back to the barn for convenient filling and freshening.
Can anyone tell me if they have done something like this and what kind of vessel they used? is it a good system? is there something I am not thinking about? etc.
Temperatures reach 100 degrees in the summer and the humidity is awful.. does anyone use a cooling system of any kind. maybe recirculating water in some way. my husband said something about recirculating through the ground to cool the water.... I don't know.

Only one MAJOR problem with that. Your water will be in a "corner". Unless you only plan on having 1 horse, somebody will get stuck in the corner and either get kicked or panick and come through the fence. When enclosing horses it is best not to have any L "corners". Try to keep your corners rounded or as open as possible, never more than 90*. Somehow, someway, someone will get hurt. Same thing with gates, its safer to have room on each side, so when the grumpy horse needs to be first the meak/mild doesn't get trapped and take you out trying to get away.
The trough under board fence works great, but instead of having it at the corner, go 2-3 sections in the fence line so they can get around it and not get stuck, just need 15-20ft hose. So although I wouldn't suggest everyone same tank, you should be fine with 2 fields. I had my troughs out in the open while in Fl both Ocala/Eustis and Flagler County and never had an issue with over heating but did have use of a barn during extremely hot weather. It does need to be cleaned [daily] but @ least 3x week. You can try putting fish in it to help with algae buildup. It does work until the birds see them, then it gets real expensive real fast.
The electric 1 1/2 tape[safe fence] comes in dark brown, which matches nicely with board, just another option.

Are you planning on using 3 pastures to rotate, or for 3 separate herds? If you are rotating, then make a 4th smaller area that accesses all 3 pastures. Use this for feeding and watering. It would have 4 gates. One for each pasture and one for humans/tractors etc. When you are using one pasture, you secure the gate open and the others closed. When you are ready to change pastures you corral all the horses and close off the one pasture as you open the others.

If you are having 3 different herds, one in each pasture, then you might need to resort to 3 troughs. You might be able to get by with two and still leave enough room for horse safety.

When I was a kid we used big old deep (cast iron) bathtubs and deep sinks for watering troughs. Because of the vitreous china around the tub/sink the water was almost always cold. The water was set up to fill automatically with a toilet valve and float (this was the old ball cock type). To keep the water clear (and prevent the horses from playing with the float) wooden legs held up a wooden platform a few inches above the lip of the tub and covered about 1/3 or a little more of the tub (so there was airspace on all sides). By covering it, there was less surface area receiving daylight and the goldfish and mosquito fish had a place to hide from predators. Overall it stayed pretty clear and we only had to clean it a couple of times a year or less. It was also set up on cinder blocks that were set up so that one block was just outside of the parameter of the tub. This kept the horses from kicking it. In a set up like this, make sure you have good drainage away from the trough. If your horses can get to the float, they will play with it until the ground is a muddy mess. Nothing is more fun that squirting someone with water when YOU are the one in charge! I used to watch the horses do this. It was a HUGE mess, but even horses that didn't like hoses and baths would squirt other horses with glee any time they could get to the float!

Obviously the biggest headache with this system is running the water lines out to the trough. After that it is pretty elementary -- even I could do it! ha ha!

I hope you can get the picture. I didn't get much sleep last night and my writing and critical thinking suffers when that happens!

See that's why I posted here. I didn't think about them getting crowded in. Thank you!

Cheryl, thank you.. we are so new at this, we need help from people that have been there and done that. A great many people in this area still use the old iron/china tubs, I've seen them. I like that idea too..

I was just thinking a bit more about this (now that I have had ample sleep LOL) and you could also some sort of filter/charcoal system to keep the water cleaner. It would benefit both fish and horses.

Also, if the water in your area has chlorine or chloramine added the fish might die... when I was a kid that wasn't a big problem, but here in San Diego they are adding more and more chlorine to the water... you can smell it sometimes. I hate it! So the water has to be treated before adding to the fish tank. Chlorine will gas off in 24 hours. Chloramine will not.

So looking up Pond Filtration in your area and what you can do about chemicals added by the water authority would be good. If you are on a well... then never mind!