Well, it could work out beautifully, but it may not. Cost aside, even though sows will usually accept other sows it's not always so. It s possible that you end up in the situation of having to return the new one, so I would check that the rescue near to you does 'dating', where they oversee the introduction and the piggies get the chance to get to know each other, and accept each other, before you take your new one home.
It's great that you are considering a rescue, but be aware that not all rescues are equal. There's been cases of people getting poorly or pregnant pigs. Not from the rescues recommended on the forum I may add. Many rescues are really good, but not all.
One extra pig brings their poop and wee output to half again, and that can have quite an impact on how often you need to clean them out. Are you happy with cleaning them more than once a week and replenishing their hay bin every day instead of every two days?.
Now by plenty big, how big? If you don't mind me asking. One person's big can be another person's small, and it's all perspective. The recommended size for three pigs is at least 5ft.
That aside the general upkeep costs shouldn't increase much. There will be an increase in bedding used, and veggies. If your pair have any beds or hidies you may need to buy another one for your new piggy.
The real shockers when it comes to cost is vets fees. You may have been fortunate in that your pair perhaps have never needed to see a vet, but a great many of our pigs do during their lifetimes, and each piggy you have increases that chance. Vets fees can run into hundreds of £, and if you are not earning your own money yet then really it's your parents who need to be prepared for dealing with that.
It's a lovely idea, taking on another piggy, and believe me there are many needing homes, but there's plenty to consider before going ahead. We'd be happy to support you with advice though