Horses establish a pecking order within their herd. Any additions to the herd can rearrange the whole line-up. The bottom horse can quickly become 2nd in command with a different leader! I had a feeling Buddy would have to give up a little rank...
It's important to introduce them slowly. You don't just open the gate and shove the new one in. They can be violent and hurt each other. And since Maggie is a senior citizen in horse years, I wanted to make sure that she didn't get injured by the younger ones. Since we got home late last night, Maggie didn't get to meet her pasture mates until today. Maggie spent the night inside Buddy's stall. Our horses only go inside to eat or if there is ice or some other weather that threatens their well-being. So Maggie had the barn to herself. (If Maggie had been boarded at a large facility or around lots of other horses, she would have been quarantined for a while before she got around the other two.)
While I was at church, Hubby moved Maggie to the round pen so she could roam around and eat grass. When I got home I opened the stalls and brought the young ones in. With Nekoda and Buddy safely locked in their stalls, I brought Maggie in on a lead. It went very well. Horses will sniff each other's breath... (Much nicer than the doggie version of greetings.) Maggie and Nekoda stayed nostril-to-nostril at the bars for a while, inhaling deeply. There was no stomping or kicking. After a bit she moved on over to Buddy's stall door. There were a couple of loud neighs & squeals, but nothing really aggressive. Neither attempted to nip between the bars. (Sorry there's no video, but working a camera at a time like this is a bad idea.)
After they satisfied themselves with exhaled horsey air, I turned Maggie out to the back pasture. She headed for that sweet spot and was out of sight of the barn door. I then took Buddy out of the paddock to the round pen. I figure I'd let him meet her later...
I put a line on Nekoda and led her to the opening to the pasture where Maggie was grazing. They both raised their heads up and eyed each other. Nekoda is not the least aggressive and I wasn't really worried about the two mares being turned loose together. I walked her part way down the pasture, then, with a quick warning to behave, turned her loose. She took off for Maggie. They sniffed each other's breath again and dropped their heads to graze. I exhaled. One down, one to go...
I left the girls in the pasture for awhile to bond. Buddy was happily munching on the overgrown grass in the center of the round pen. (It's not had a lot of use while I've been sick, so it looks more like a pasture than a work-out ring.) Occasionally, Nekoda would run back to the barn gate to yell over to Buddy. They don't like being separated! But then she would return to the grassy end with Maggie.
After a while I decided it was time to introduce Maggie and Buddy. I brought Nekoda back into her stall. She protested loudly! With Maggie at the other end of the pasture, I walked Buddy toward her. Buddy is pretty laid-back, but I wasn't about to take him up to Maggie on a lead. That could be dangerous. I would be just close enough to split them up if needed. Buddy was very attentive to Maggie, but seemed pretty calm as we got closer. I finally reached the end of my comfort zone and removed Buddy's halter. He trotted over to her and they sniffed... He smelled her neck... She squealed at him... He backed off... They started grazing! Wow, that was easy! I watched them closely for awhile. They stuck together, but respected each other's area.
By now, Nekoda had had just about enough of being shut in the stall. I opened the door and let her out. She took off for the grassy end of the pasture and I went for my camera.
And again, things are as they should be...
And again, things are as they should be... 
