What is first stable product in $C_3$  cycle? 

The first stable product during the Calvin cycle is 3-PGA(3- phospho glycerate).

The first step of the Calvin cycle is Carboxylation:Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilized for the carboxylation of RuBP.

During carboylation,  fixation of CO2 into a stable organic intermediate takes place. 

 This reaction is catalyzed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.