as long as it is up high and out of reach, and unloaded, I wouldn't be too worried, especially about a 3 year old. For one, even if he got a hold of the gun, how is he gonna chamber a round? I have an 8 year old and he can't even load an AR-15. Can your wife even load a round into it?
By the time they are old enough to start locking them up, they should be trained on how to use and handle it safely anyway.
I am pretty good about locking up my guns, but if I left one out for some reason, I wouldn't too freaked out. Liberals will cry about this but gun safety is more about education than locking things up. Teach your kids how to handle a gun and to respect it.
I have a brother who has guns all over his house. Always has. In fact, everyone where he lives does that. It's far more common in certain areas of the country than most city dwellers (usually liberals) realize.