To determine how long the EcoFlow River 3 can operate a Dometic CFX28 Coolbox with a power consumption of around 40 watts, we need to calculate the capacity of the River 3 and the energy requirement of the Coolbox.

The EcoFlow River 3 has a capacity of 245 Wh (watt hours).

The Dometic CFX28 Coolbox consumes around 40 watts. To calculate the operating time, divide the capacity of the River 3 by the power requirement of the Coolbox:

\[ \text{operating time} = \frac{\text{capacity of the River 3}}{\text{power requirement of the Coolbox}} = \frac{245 \, \text{Wh}}{40 \, \text{W}} \approx 6,125 \, \text{hours} \]

The EcoFlow River 3 can therefore operate a Dometic CFX28 Coolbox for around 6 hours and 7 minutes.