I'm not really clear what you're going to do. Divide the 3 acres into three; Year 1, keep the three ponies on one acre, say Acre 1, over summer then what if they have to be out all year 24/7. Acre 2 keep weaners? Acre 3? then Year 2 pigs in Acre 1, ponies in Acre 3, and whatever was in Acre 3 in Year 1 on to Acre 2, where the pigs were? Just trying to get a picture of your plan
If you are currently not grazing or cutting an area all summer, the grass will be seeding and the dead grass will be building up thatch. You should harrow to remove dead grass, moss etc in spring at least. Like using a lawn rake, it gets all the dead grass out and allows light and air into the roots of the coming grass. Rolling may or may not be necessary - I'm relying on the livestock to "roll" the land.
Grazing or cutting before seeding improves pasture. Grazing and cutting encourages the grass to tiller, which is vegetative reproduction, rather than seeding, leading to a denser and more resilient sward. Sheep are very good for improving grazing - their hooves also act as rollers, keeping the soil in contact with the roots and of course, the dung provides fertiliser in small packages.
You may be able to get seed mixes suitable for wetter land - it will need to be robust to stand up to the inevitable poaching if stock are going to be on the land in the wetter months.Clover in the seed mix will add nitrogen - use small leafed native clovers, not large leafed white clover or red clover.
Weaners will not improve the grass - they will destroy it. If you want to have permanent pasture suitable for ponies, then you don't really want to plough and reseed every couple of years, which is effectively what the pigs will do, or the grass will never get a chance to properly establish.
Does this help or just make it worse?