hexagon in the center of the supercell. The relaxed structure, depicted in Fig. \[Vac1\]c, creates [*55-10*]{} defect consisting of two pentagons and one decagon. The band structure and DOS of a graphene with a tri-vacancy defect is presented in Fig. \[Vac1\]c. Tri-vacancy introduces four defect levels, two of which narrows back the band gap and other two levels cross the Fermi level, transforming the graphene to metallic state. Apart from the tri-vacancy defect two other structures with three vacancies are considered, which are produced (i) by removing two nearest-neighboring carbon atoms in a hexagon but the third one is located in the next-nearest to these two atoms as it is depicted in Fig. \[Vac3\]a, producing three pentagon and one undecagon defect, (ii)by removing three carbon atoms in a hexagon all belonging to one sublattice as it is shown in Fig. \[Vac3\]b, which produces three nonagon and one triangle. All these configurations with three vacancies contain dangling bonds, and therefore, reveal a magnetic moment. Nevertheless the formation energy and the magnetic moment of the tri-vacancy of [*55-10*]{} defect take the values $8.84~eV$ and $1.00~\mu_B$, correspondingly (see, Table \[table2\]). The values of the formation energy and the magnetic moment for other configurations 