In Fehling’s solution, the reaction between copper(II) ions and aliphatic aldehyde is represented as;
RCHO+2Cu2++5OH−→RCOO−+Cu2O+3H2O
Hence ,Acetaldehyde (CH3CHO), Methanal (HCHO), and cyclohexane carbaldehyde

compounds gives positive Fehling's test.
Hence, the answer is 3.