I think there is no difference between gpu=2 or 3.
In my experiment:
batch-size=8  gpu=2  -->batch_size=4 for single gpu
batch-size=8  gpu=3  -->batch_size=2 for single gpu(so total batch_size is 6)
batch-size=8 or 6, under normal circumstances, it does not have much impact on performance
For some task which are very sensitive to batch_size may need to take it into account