That is an excellent question. Multiple people invented 3D printing technology around the same time. Some articles on the internet state that Chuck Hull was the inventor of 3D printing. I did some research on the patents in this area and here are my findings.
Indeed the first commercial machine was offered by 3D Systems — company founded by Chuck Hull — in 1986. Based on internet research he seems to have invented the machine in 1984 but waited to file a patent for stereolithography (SLA) until 1989.
Carl Deckard filed for a patent on Selective Laser Sintering (SLS) in 1986. The same year Chuck Hull commercialized SLA technology.
But Ross Housholder already filed a patent in 1979 for “A molding process for forming a three-dimensional article in layers”. The patent was never commercialized, but it was referenced in the patent of SLA by Chuck Hull.
So to me Ross Housholder is the inventor of 3D printing.