Those are reportedly trial production units, built as part of the preparation for series-production.
It's believed that those are trial units, painted and assembled before the company will start series production.
"From the pic we can see at least 8 MIC Model 3 are under trial production inside Tesla Shanghai Gigafactory GF3 "
Usually, manufacturers are building trial cars to calibrate the production lines, double-check the process and train employees.
In the case of Tesla, some sources report that production might start on October 14 or so, using LG Chem batteries.
Who knows, maybe soon we will see the first made-in-China Model 3 Standard Plus driving around for validation tests?