Are they all castrated? If any of them aren't then you might need to rethink
In the case that they are it should be fine after the initial settling in/pecking order establishment.
Keep the new one away from the other two for (ideally) three weeks until you know they are disease and parasite free. Once the isolation time is past and you've made sure their tusks are all short/trimmed. I'd personally just let him in with them - with an acre there's plenty of room for him to escape them. Put in a second area for sleeping (it should just be needed for a couple of days), introduce them at evening feed (less time to fight before bedtime), give them a couple of cans of beer to chill them out a bit. I know it sounds strange, but it'll help - promise! Make sure they have plenty of water and keep an eye on them. There WILL be fighting, it will look brutal and you'll want to step in and separate them - DON'T! Providing it's all pushing, shoving and head/shoulder focused it will settle in a couple of days - if they're biting at each others back ends, it's more serious. Have a can of antibiotic spray ready for bites and scrapes and general war wounds.
I mixed my senior boar with some sows and gilts yesterday (he's a BIG lad) one of the gilts took a bit of a dislike and actually back flipped him through the field - he let her carry on for five minutes or so before he showed her he was the boss
Males and females usually mix well, youngsters and castrates are usually easy but sows.....wow they can be viscous!
Good luck!