 Solution

A has a strong ester C=O band at roughly $1715$–$1730\mathrm{c}{\mathrm{m}}^{-1}$, lowered by conjugation with C=C. B's unconjugated ester carbonyl is nearer $1735$–$1750\mathrm{c}{\mathrm{m}}^{-1}$; it also has terminal-alkene features, including the approximately $990/910\mathrm{c}{\mathrm{m}}^{-1}$ pair.

C has no carbonyl band. Its terminal alkyne gives a relatively sharp sp C-H stretch near $3300\mathrm{c}{\mathrm{m}}^{-1}$ and a usually weaker C≡C feature near $2100$–$2150\mathrm{c}{\mathrm{m}}^{-1}$. C-O stretching alone is less diagnostic because esters and the acetal all contain C-O bonds. The numbers are characteristic ranges, not exact frequencies inferred from an unprinted IR trace.