Most portrayals of Romeo and Juliet in art focus on the balcony scene or the deaths of the lovers. In Act III, the lovers do not appear together, but instead we have Romeo in dialogue with Friar Lawrence, Juliet with her nurse, and duel scenes. Portraits of either character alone could also be used to represent this Act.
Charlotte and Susan Cushman (the Cushman sisters) in Shakespeare's Romeo and Juliet in 1846. An 1846 painting found in Shakespeare: The Globe & The World ISBN 0195026462 shows Juliet and her nurse.
"Juliet" or "The Blue Necklace" by John William Waterhouse. (1898) shows Juliet alone and pensive.
The 1867 water colour costume picture, Roméo by Bianchini, shows Romeo with his sword, which fits well with the duel theme.