2 things, any vehicle that has a gun in it should be locked when not occupied.
A 3 year old should be supervised.
She obviously wasn't riding her bike when it happened as she would have had to open truck door and enter it. Where were the adults?
Agree with your first point.
Second and third points I disagree with, having had three 3 year olds of my own. Sure, kids should be supervised, but it's impossible to keep track of even one 3 year old at all times. Take your eyes off them for 30 seconds and they find mischief.
Little kids are like skillful, sneaky, burglarizing monkeys - curious climbers who open supposedly locked doors, locate and discharge weapons, wield sharp objects, pull furniture down on top of themselves, drown in toilets, close themselves into discarded refrigerators, insert screwdrivers in outlet holes, place plastic bags over their heads, drink cleaning potions, etc.
That's why adults baby-proof houses and vehicles.
Lethal weapons, knives, medications, cleaning products and other dangers are supposed to be inactivated or fully secured at all times by the owners - and not just placed in a high cabinet.
Adults are legally and morally required to exercise due diligence to help ensure little children aren't injured or killed because of our neglect.
I think the responsibility of this accident falls squarely on the shoulders of the gun owner.