After explaining general characteristics such as overspecification, found in the diagrams of Greek manuscripts of Euclid’s Elements, diagrams in some propositions of Book III are examined in detail. Codex P (Vat. gr. 190) and b (Bologna) are common in avoiding overspecification in a couple of propositions. However, further examination of diagrams of Book III in other manuscripts including those in the Arabic tradition, and collation of the text suggest that the common feature in the diagrams of codex P and b is rather due either to independent efforts to avoid overspecification or to contamination of traditions. It is codex F (Florence, Laurenziana 28.3) that most often coincides with codex P in Book III.