Three Part Divisi in Two Violin Sections
In a situation where you want to spread out three voices over both violin sections, usually the better choice is to divide the second violins into two voices and leave the top voice to the first violins alone. Even though this might seem against logic (with the 1st violin section usually being bigger than the seconds) but there are two reasons for doing so.
The first one is to highlight the top note by putting the whole first violins on it as usually this line has a leading melodic quality that should not be underbalanced. The second and more important reason is that the higher you get with violins the more instruments you need to sustain a substantial string sound that doesn’t become thin. With small string line-ups like for instance eight 1st Violins you wouldn’t want to split the first violins into two parts and possibly having only four violins on the top line. So the preference is always to get as many violins as possible on the top line to ensure enough substance of sound.
Again, this is a rule of thumb. Depending on context it might be a better choice to pick other divisi options, e.g. splitting both sections into all three voices – which would be suitable in mid-register, soft dynamic, long sustain situations or when two of the three voices move in octaves to put these octaves into one section (reinforcing harmonics) and the remaining voice into the other. The general impulse however should be to have as many instruments as possible on the top line and make decisions based on this premise.