There are #3/13xxN_A" carbon atoms"#, where #N_A="Avogadro's Number, "6.022xx10^23*mol^-1#
Explanation:
In #13*g# of #""^(13)C# there are precisely #"Avogadro's number of atoms"#.You have specified #3.00*g#, thus there are #(3.00*g)/(13.00*g*mol)xx6.022xx10^23*mol^-1##=##??""^13C " atoms"#