The oxygenation activity of RuBisCO enzyme in photorespiration leads to the formation of :

In ${\mathrm{C}}_3$ plants some ${\mathrm{O}}_2$ does bind to RuBisCO, and hence ${\mathrm{CO}}_2$ fixation is decreased. Oxygenation activity of RuBisCO produces one molecule of 3C compound PGA, and one molecule of 2C compound phosphoglycolate. 

During the carboxylation step of Calvin cycle, RuBP combines with ${\mathrm{CO}}_2$ to form 2 molecules of 3PGA (3C), which is catalysed by RuBisCO.