What payment terms means that payment is due in 30 days but the payer will get a 3 percent discount if payment is made within 10 days?

3n10 net30

This is usually the way it's stated (or something similar)

2n10 net 30 means a 2 percent discount if paid within in ten days of the invoice date with full payment due in 30 days. So 3n10 would describe a 3 percent discount.