The Mercedes-Benz GLB is the second compact crossover in the carmarker's range, and will sit - you guessed it - between the GLA and the GLC. However, it will have one key advantage over its two stablemates and its rivals like the BMW X1, Audi Q3 and Volvo XC40, and that is a third row of seats. We walk you around the new crossover to tell you what you can expect.