For three sows, you would want to aim for about 13 square feet. Assuming the cage is 110cm x 110cm, this comes in at about 12 square feet. Ideally, you would want to expand your cage to a 2x5 C&C cage or larger. Because you already have a 1x3 loft, you could consider bringing it down at adding it to the end to make a 4x3 C&C cage, which would be a great size for three girls and would mean you wouldn't have to purchase more materials. I did something similar when I adopted a third piggy. I had a 2x4 C&C cage with a 2x2 loft, so I ended up taking it down and making a 2x6. My current cage is a 4x5 made of a 2x5, 2x4 and 2x2 base taped together (you can see in this photo where I joined it).
Hope this helps!
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