there is no question that their original intent was defense against tyranny, national defense and self defense. However, aside from the last reason listed i personally don't believe the other two are realistic today nor is it constructive to constantly claim otherwise. The disparity between civilian weapons and government owned has grown infinitely since the days of muskets.
The general perception of those who run around saying their guns are there to defeat the government "when the time comes" is not positive and does far more to move the general public against gun rights rather than in favor of them. The worst is the small but loud group of people living in fantasy land who are today calling on people to "get ready" for the coming revolution. The irony is that the use of force by a small minority to overthrow democratically elected representatives is by definition tyranny.
I'm not claiming that it impossible for tyranny to reign in america but only that us and our AR15's aren't going defeat a modern, technologically advance military. And before anybody says it, the situation in Afghanistan is absolutely not evidence to the cntrary. Making it too costly to continue an occupation thousands of miles away for years on end is not the same thing as defeating a military at home.
Regardless of intent though, the second amendment is law and not going anywhere. The reasoning behind the third amendment isn't really applicable today but it is still law.
Actually, the threat of armed rebellion, or large scale insurrection, does tend to put limits upon Federal or State action. No one wants to start a civil war, and the threat of one will cause popularly elected government officials to think twice. It may still happen, but they will think twice about it.
Chances are very good, that if armed resistance against tyranny is supported by a large portion of the populace, the military will either be split with some backing the freedom fighters or will elect to sit out the fight, as happened in the War of Northern Aggression.
We don't need to defeat a NAZI style occupation force, we only need to convince our children, brothers, sisters, and friends in the military, that they are backing a tyranny. Or at least, force the military to acknowledge that they are indeed backing a tyrannical regime.
How many Army or Air Force officers do you know that would be willing to use nuclear weapons against their own fellow citizens?
Your attitude that any such attempt is doomed to failure, is not supported by history, either ancient or recent.
By the way, if you read the reasoning behind, and the history of the 3rd Amendment, you could understand how it might happen again. It wasn't just about forcing troops to live in the homes of private citizens, but forcing the communities to pay for the cost of stationing such troops among them.