An application for a State Pension can't be backdated by more than 12 months. So the lady you refer to can't simply demand to be paid three year's worth of pension as a lump sum now.
She has deferred her State Pension so, after 3 years, she's now in a position whereby (assuming that she doesn't want to continue deferring her State Pension any longer) she can either:
(a) apply in the ordinary way now and benefit from the increased payments that she'll get from having deferred her pension by three years ; or
(b) submit a claim which is backdated by 12 months. She'll then get increased payments based upon just two year's deferral but with one year's worth of pension (paid at that same rate) given to her as a lump sum.