I would most highly recommend the "WeatherTech" molded plastic mats made for the I3. They fit perfectly and are very easy to clean. Also guarantees that you will not have "spills" of liquids. I do not use the back seats at all so I just bought the one piece that fits over the two front floors and the middle "hump" and used the BMW ones for the back seat area. However, I did also buy the same mat for the back trunk area and it has protected it perfectly over the year of ownership. But, they are expensive.
2019 i3 BEV, Capparis White, Tera World, Harmon Kardon Audio System, 19 inch 429 wheels