In $C_3$ plants, the first stable product of photosynthesis during the dark reaction is :-

In the Calvin cycle (dark reaction) of $C_3$ plants, the $CO_2$ acceptor is ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound.
The enzyme RuBisCO combines $CO_2$ with RuBP to form an unstable 6-carbon intermediate, which immediately splits into two molecules of 3-phosphoglyceric acid (PGA):
$RuBP + CO_2 + H_2O \xrightarrow{RuBisCO} 2 \times 3\text{-}PGA$
3-PGA is a 3-carbon compound, which is why these plants are called $C_3$ plants.
Phosphoglyceraldehyde is formed later by reduction of PGA. Oxaloacetic acid is the first stable product in $C_4$ plants, and malic acid is important in $C_4$ and CAM plants.
So the first stable product in $C_3$ plants is 3-phosphoglyceric acid.