Hi and welcome to the forums, krisus :)
Although Samsung may have confirmed that the phones are or could pass for being authentic, these types of products are still inevitably considered counterfeit since they are not sold in the proper channels. The manufacturer you purchased your phones from may be the official manufacturer for Samsung however they are not licensed to sell these in the market so they cannot sell the items as Samsung Galaxy S3 rather as an OEM phone.
Many branded products are actually manufactured in China and many of those sold as knock-offs actually come from the official or licensed manufacturers who are not licensed to distribute them - thus the products are sold and considered knock-offs.
But like Marc said authentic and China are rarely used in one sentence. So best to avoid China based suppliers selling authentic
brands to be on the safe end always :)
All the best!