call MatCreate(PETSC_COMM_WORLD, this%A, ierr)

        call MatSetSizes(this%A, lm, lm, M, M, ierr) ! lm is the local size of the matrix , M the global one

        call MatSetType(this%A, myType, ierr) ! myType is MATMPIBAIJ
 
        call MatSetBlockSize(this%A, 4-bdim, ierr) ! 4-bdim is equal to 3 in this case

        call MatMPIBAIJSetPreallocation(this%A, 4-bdim, flubioSolvers%d_nz, mesh%d_nnz, flubioSolvers%o_nz, mesh%o_nnz, ierr) ! d_nnz and o_nnz is the number of diagonal and off diagonal non zero blocks

        call MatSetUp(this%A, ierr)

        

       ... some non relevant code .....

         call MatSetValuesBlocked(this%A, 4-bdim, mesh%cellGlobalAddr(iElement)-1, 4-bdim, mesh%cellGlobalAddr(iElement)-1, blockValues, INSERT_VALUES, ierr)  mesh%cellGlobalAddr(iElement)-1 is an integer equal (not an array) to the block element number the block matrix blockValues (3x3) belongs to.

Any evident errors?