These are minimal answers without detailed explanations. Please see me for a complete explanation of each answer. Affects Na action potentials by increasing duration only since AP height is determined by other parameters. Different post-synaptic receptors in cells 1 and 2 and cell 3 has both. No, due to Na inactivation of the Na channels which just opened.
Dendrites are devoid of voltage dependent channels necessary to produce an action potential.
Because the purpose of dendrites is to summate and integrate multiple signals from multiple pre-synaptic sources.