3Dfx's mistake was to try to produce the chips along with the PCB etc from the Voodoo 3 onwards. The earlier Voodoo products were made by the likes of Hercules & Creative with the graphics chip sourced from 3Dfx.
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They delays of the "Rampage" project also hindered them further. They struggled to get the Voodoo 4 & 5 to market and they ended up arriving almost at the same time and due to the delays, were already behind the Geforce 256 as they were aimed at the TNT2 era.
The Rampage was to compete with Geforce 256/Geforce 2 but due to the delays 3Dfx decided to keep bolting more & more VSA-100 chips together and go with brute force until Rampage was released. The Voodoo 5 6000 never made it to market though due to many problems with drivers etc.
The loss of revenue eventually caught up to them and they declared bankruptcy with Nvidia buying the majority of their assets and a lot of it went into the Geforce FX series cards.
The demise of 3Dfx left only ATI, Nvidia & Matrox as the main players, with Matrox quitting the "gaming" market after their failed Parhelia range of cards.