Yes Sir, you may do so. You may also request some other co-pax in the 2AC Coach to shift to your seat in 3AC, or someone in your coach in 3AC to shift to your co-pax's seat in 2AC, so that you can be with your co-pax throughout the rest of the journey.
.
In either case, you can sit with your co-pax, as it is 2AC, and so only 4 people in a coupe. But, for sleeping, you may have to request to swap your seat with some other co-pax, preferrably someone...
more... travelling alone, as they are the ones who might agree.