For an abelian category A, we define the category PEx(A) of pullback diagrams of short exact sequences in A, as a subcategory of the functor category Fun(∆, A) for a fixed diagram category ∆. For any object M in PEx(A), we prove the existence of a short exact sequence 0→K→P →M →0 of functors, where the objects are in PEx(A) and P (i) ∈ Proj(A) for any i ∈ ∆. As an application, we prove that if (C, D, E) is a triple of syzygy finite classes of objects in mod Λ satisfying some special conditions, then Λ is an Igusa-Todorov algebra. Finally, we study lower triangular matrix Artin algebras and determine in terms of their components, under reasonable hypothesis, when these algebras are syzygy finite or Igusa-Todorov.