The resonance structures of BF3
are so unstable that you mustn't draw them. Under normal circumstances, when the contributors are relatively stable, you draw them all. All of these contributors would be that particular compound.
A compound is not one contributor one minute and another the next. It's all of them at once. We just can't depict this. Usually, dashed bonds indicate this reality.
, however, does not really have any resonance contributors. You can draw these contributors and see how unstable they are, and therefore ignore them. This implies that BF3
follows the general structure indicated by VB theory. A very general explanation of VB theory