This work is one of a number of recent works aimed at investigating the representation theory of blocks of group algebras \(k{\mathfrak S}_{n}\) of symmetric groups over fields of prime characteristic. Here the author is interested in blocks of weight three in characteristic three. By work of \textit{J. Scopes} [J. Algebra 142, No. 2, 441--455 (1991; Zbl 0736.20008)], there are up to Morita equivalence 12 such blocks, some of which are conjugate. The Ext-quivers of seven of these have been computed by previous work of the author [Math. Proc. Camb. Philos. Soc. 133, No. 1, 1--29 (2002; Zbl 1009.20014)] and work of \textit{K. M. Tan} [J. Pure Appl. Algebra 153, No. 1, 79--106 (2000; Zbl 0986.20013) and Math. Proc. Camb. Philos. Soc. 128, No. 3, 395--423 (2000; Zbl 0993.20011)]. Of the remaining five blocks, there are two sets of conjugate pairs, so one is reduced to three blocks. The author computes the Ext-quivers of the remaining blocks by investigating certain \([3:2]\)-pairs with the aid of Schaper's formula and his previous work. The most difficult case involves computing extensions for the simple module \(D\) having partition \(\langle 3,3,3\rangle\) in \(\langle a,b,c\rangle\)-abacus notation. Beyond the Ext-quiver computations, the author also computes the submodule structure of the projective cover of \(D\) and finds that its lattice is distributive. This example may suggest some general properties of such projective covers for blocks having weight at least the characteristic of the field.