The first stable product of photosynthesis during the dark reaction in C3 plants is 3-phosphoglyceric acid (3-PGA). This process is also known as the Calvin cycle or the light-independent reaction. Let's break down the explanation into sections:

1. Introduction:
Photosynthesis is the process by which plants convert sunlight, carbon dioxide, and water into glucose and oxygen. It occurs in two stages: the light-dependent reaction and the light-independent reaction. The light-dependent reaction takes place in the thylakoid membrane of chloroplasts and produces ATP and NADPH. The light-independent reaction occurs in the stroma of chloroplasts and uses the ATP and NADPH produced in the light-dependent reaction to fix carbon dioxide and synthesize glucose.

2. Dark Reaction in C3 Plants:
C3 plants, such as wheat, rice, and soybeans, are named after the first stable product of carbon fixation, which is a three-carbon compound called 3-phosphoglyceric acid (3-PGA). During the dark reaction, carbon dioxide enters the stroma of the chloroplast and combines with a five-carbon compound called ribulose bisphosphate (RuBP) to form a six-carbon compound that immediately splits into two molecules of 3-PGA. This reaction is catalyzed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase).

3. Formation of 3-Phosphoglyceric Acid:
The enzyme RuBisCO combines carbon dioxide with RuBP, resulting in the formation of an unstable six-carbon compound. This compound quickly breaks down into two molecules of 3-PGA. Each molecule of 3-PGA contains three carbon atoms and one phosphate group. The phosphate group is derived from ATP, which is produced in the light-dependent reaction.

4. Role of 3-Phosphoglyceric Acid:
3-PGA is the first stable product of the dark reaction and serves as the starting point for the synthesis of glucose. It undergoes a series of enzymatic reactions to form glyceraldehyde-3-phosphate (G3P) and other intermediates, which eventually lead to the production of glucose. G3P can also be used to regenerate RuBP, allowing the dark reaction to continue.

In conclusion, the first stable product of photosynthesis during the dark reaction in C3 plants is 3-phosphoglyceric acid (3-PGA). This three-carbon compound is formed when carbon dioxide combines with ribulose bisphosphate (RuBP) in the presence of the enzyme RuBisCO. 3-PGA plays a vital role in the synthesis of glucose and the regeneration of RuBP, allowing the dark reaction to sustain itself.