A happy BP is a hiding BP. If your ball python has the right setup, they'll be hiding all day and you'll see them only roam at night. Because of that, there's not really a lot of "natural behavior" to observe, unless you're awake all night. The most economical, as far as keeping a lot of snakes for not a lot of cost for caging or size, a rack is the way to go. To get terrariums (I don't recommend glass tanks, they're difficult to keep properly, you usually have to sacrifice either heat or humidity) enough for at least 4 snakes, you would need a minimum of 2 Animal Plastics T8s divided into two (or something of a smilar size). At $150 each just for the cage itself, under cage heating, a radiant heat panel on each side (for ambient temps), regular lighting if you want to see inside (optional), as well as thermostats that could handle two heat sources, like a Herpstat 2 (would need two of these) or a Herpstat 4 (could handle both cages). You'd be looking at ~$500 a cage, not including any costs for hides, bedding, any decorations if you would like. The upside of all that cost, is that with that size of a cage, you can keep a BP from hatchling up to adult. Unless you end up with a really huge female (4000+ grams) you can keep a snake in one-half of a T8 all its life. To give each snake its own cage the price would scale up, of course. And if you get two males, you would need a third cage along the line.

Compare it to a rack: Something like this would hold your 3 girls all their life (you would have to clutter it up for when they're babies) and you would just have to buy another rack for your males later. With that, you're looking at $300 for the rack itself, and $110-$115 for a Herpstat intro+ (the bare minimum, I would not recommend a Herpstat basic) or a Herpstat 1. So for terrariums you'd be looking at $1000 at a minimum and for a rack you're seeing ~$450 plus an upgrade later if you get more snakes. The thing about a rack though, is that you have to keep the ambients in the room up, so if your house is cold, you will probably have to run a space heater which can run up the heating bill.

I use racks exclusively. I have never kept a BP in a terrarium or tank. I have one snake (she's actually my father's) who I would love to set up in a T8 once I get the money. She is a big snake that can't be bred (well she could, but it would pose serious health risks for her) and my dad loves her. She's just fine in a rack, but because she's his and just a pet, she's the only one I would think about setting up a big decorated enclosure for and keep her as a "display" animal, insofar as a BP can be a display animal.