All OBA counters also have "3" in a black hex icon. Could not find explanation in rules.
The number in the black hex describes how many adjacent hexes are affected by the OBA. In this case, it would be the target hex and 2 adjacent hexes.
In the case of biologicals, only the target hex would be affected.
Figure 2 on page 14 of the rulebook shows an example of the different results from OBA.