Forrest:
I strongly recommend that you purchase and read the following two books:
1) Massad Ayoob, "In the Gravest Extreme"
2) Andrew Branca, "The Law of Self Defense"
Both are available from Amazon. If possible, I recommend taking a concealed carry course, e.g., Ayoob's LFI-1 or equivalent.
It is vital that you understand the laws regarding the justifiable use of deadly force. If you don't understand the laws, you could wind up in the grey bar hotel, being introduced to your new roomate, Mongo.
As others have pointed out, you, or another innocent other, must be in immediate danger of death or grave bodily injury. In most states, if you are outside of your home or place of business, you must retreat if it is safet to do so.
Note that you must be an innocent party. If you have done anything to provoke or inflame the situation, then there is a good chance that you will not be allowed to claim self defense. For example, suppose you're walking along with your wife, when some guy on the sidewalk starts making crude comments to her. You tell him to watch his mouth around your later. Words are exchanged, which leads to shoving, which leads to fighting, he pulls a knife and you pull a gun. It's quite possible in a situation like this that you would not be allowed to claim self defense, since you could have kept your mouth shut and just walked away.
In your home, you don't have to retreat. That said, you may not have the right to shoot someone inside your home unless you are under threat of death or grave bodily injury. If a burglar is headed out the door with you TV, in most states you couldn't shoot him. Note as well that in most states you have to retreat when outside your home, even when on your own property.
Realize as well that coming to the rescue of a third party can be very risky, simply because you may not know what is really going on. Suppose two men come running out of a store, the second one carrying a gun and chasing the first. Is the first man the store owner, fleeing a killer? Is the first man a thief, being pursued by the store owner. Is the first man a felon, being pursued an undercover officer?
To address your three situations, you have to consider ability, opportunity, and jeopardy.
For the baseball bat, yes he has the ability to kill you. The questions are 1) does he have the opportunity and 2) have his actions put you in jeopardy? If he is close to you, then yes he has the opportunity. If you are in your car and can drive away, then he may not have opportunity.
If it is you against 4-5 guys, then as others have pointed out, disparity of force comes into play.
If someone is going to run over you with a car, I would strongly suggest that you try to dodge the car, rather than try to shoot the driver. It is quite hard to shoot a moving target and it is quite easy for your shots to ricochet off the car if you are at any angle to it. And even if you shoot the driver, the car isn't going to stop immediately, so you are still going to have to dodge it.
M1911