The number one thing you need if you are getting horses is knowledge. Do you know anything about horses? If not, see if you can put off the delivery until you can at least learn how to care for them. Number two thing is at least three acres of pasture/paddock, since you said you are getting three horses. Some disagree and say that's too much, but IMO if a horse can't break out into a full gallop and fly like it was born to in its pen, the pen is too small. I have three horses myself who live on about, oh, 6-10 acres of land, perhaps? Number three thing: something to put water in. If you don't have a pond or lake on the property you are keeping them on, you'll need a pretty big one. Our horses simply have some buckets spread around but they also have a lake. If they get thirsty they'll go. Pistol will show them the way. :P So, number four thing: grass in the pasture. If they don't have grass in the pasture, that really stinks. If you don't , I suggest planting some- though your horses may trod on the seeds and mess it up anyway. If you don't have grass, you'll have to spend most of your money on hay. You would need a huge round bale likely. And you would have to muck it out if it's only just min size (one acre IMO) and certainly if it is less. Number five: TIME. Enough said.
Number six: Grooming supplies. This includes a currycomb, stiff/dandy/hard brush, soft/body brush (optional but suggested- however, don't use a soft brush all the time if your horses will have the option or simply will be out in the weather), face brush (optional but suggested), mane and tail comb(s) (optional, suggested if going to show or if you are OCD about that kind of thing like me), leg brush (optional), and most importantly, a hoof pick. You will probably want something to hold all this stuff in too. Oh, and you'll need one of each of those for each horse (I.e. 3 currycombs, 3 dandy brushes, 3 hoof picks...etc) Number seven- IF you are ADVISED by a vet/horse person who seems like they know what they are doing, you may want to get grain for your horses.
Number eight- You need to know what your horses are at their previous home. If you wish to continue them on the same food you will need THAT kind of food. If you want to change their food in any way, either ask the old owner for some of their food they had been eating- enough to get them worked off it for seven to ten days- or do the slightly less awkward thing and buy some of the old food and the new food. You will have to introduce it slowly over seven to ten days- more or less. I suppose for small changes, five or six days would be okay, and I don't see any reason that making the change 10+ days would hurt.
Number nine- this may be the last one but it is still important though I have these listed (or attempted to) by order of importance. You will need three halters and three lead ropes.
As an afterthought, if you are going to ride them, you'll want tack but that can wait. This is a big undertaking.
Best of luck,