Which of the following statements regarding the mechanisms of neuronal communication is/are correct?

1. The resting membrane potential of a neurone is maintained partly because the axon membrane is significantly more permeable to potassium ions ($K^{+}$) than to sodium ions (${Na}^{+}$).
2. During the relative refractory period, a stronger-than-usual stimulus can trigger a new action potential because voltage-gated sodium channels have recovered from inactivation, but the membrane is hyperpolarised.
3. Spatial summation occurs when a single presynaptic neurone releases neurotransmitter repeatedly in a short period to exceed the threshold potential in the postsynaptic neurone.