I suppose it depends on the meaning of “adequate source.”
In a diet where we are overwhelming ourselves with ω-6 fatty acids, an “adequate” amount of ω-3 would be difficult to achieve. (Since they are both handled by the same cell receptor, they need to be ingested in about equal proportion in order to have an equal chance of making it into the cells, or so I understand).
On the other hand, while ω-6 and ω-3 fatty acids are essential to human health, I suspect the necessary quantities of either are actually fairly small. (I’m sure someone has worked it out, but I’ve never seen the study.) So in the absence of excessive ω-6 intake, it might be possible for a reasonable amount of butter to supply an adequate amount of ω-3.
Just going by my personal N = 1, I eat a lot of butter and avoid seed oils, and I seem to be fine.