I hope I've got this right - you seem to be saying your Basil is growing in the same pot as Rosemary, Sage and Thyme, and that its been in the same pot for three months. If I have got that right, its not really surprising that your Basil is having trouble thriving. Basil is an annual plant; Rosemary is a large, evergreen shrub that wants to get 6 x 6 feet, Sage is another shrub that makes about a foot tall and twice as wide, French Thyme is a slow growing sub-shrub. All take up root space, and are, by now, depriving your Basil of its root room.
All these should be growing in separate pots, but in regard to your Basil, that too deserves its own pot, though the season is getting on a bit now to try splitting and repotting them all from the Basil point of view, given its only an annual. I think I can see young Basil leaves growing lower down, so maybe just cut back the woodier stems and hope that the Basil doesn't lose the unequal struggle to survive in the same container as all the other plants.