Can I connect a 2 x 10 cab rated at 400 watt @ 8 ohm and a 1x15 cab rated at 400 watt @ 8 ohm to the BH800?
Yes it is possible. As the minimum impedance load for the BH800 is 4 Ohm.
Connecting 2x8 ohm cabs in parallel, you will reach the minimum load of 4 ohm. Regarding the power, please see the BH800's tech specs below:
This will not be a problem as the power handled by the BH800 will be divided into each cab.