by sigmazero13
First, you don't need to spend 3 influence for either the hero or a new card. You can spend UP TO 3 influence to buy a new hero with a Rally Support action; you then draw that many hero cards and pick one. So you could just spend 1 influence and just take what you get.The same applies to buying a new commander card - you could spend 1 and just draw 1 card. However, this rule only applies when replacing your current commander card.
Note, however, that when your commander dies, you don't immediately get a new commander, regardless of whether you have other heroes. At the beginning of your Quest phase, if you don't currently have a commander, you can promote one of your heroes... but only if you have a hero at a Stronghold. In that case, you simply make that hero a commander, and draw a card for it - no spending required.
Thus, in the situation you describe, and you lose your last hero, if you use Rally Support before the next Summer, you can get a hero, and as long as the hero hasn't left the Stronghold before Summer, you can then promote that hero, at no cost, to be your commander.