I travel a lot but I only carry a single carry-on with my laptop, camera and cellphone. I already have an external USB battery pack for my cellphone but I cannot charge my Macbook, nor my Nikon D3 with it. I was wondering if it was possible to buy an external battery pack - something like a power adapter except it also stores power and has not only USB outlets to charge my cellphone but also AC outlets to charge my laptop. Is it legal to carry such a battery in my carry-on? Is it too heavy? What are some recommended products or brands? I searched on Amazon and I could not find such a thing (only found results to microUSB charge cellphones). What term should I search for on Amazon?
Okay, so I understand your issue and considering that you need to charge your laptop through a battery pack you will need a battery pack which can output a voltage higher than 5V which is the USB standard.
For your Nikon D3, the battery is 11V and the standard charger provides 12.6V at 1.2 A in order to charge the battery.
Now, one option which you should immediately rule out is expecting to keep a device which would provide 100V to 240V AC voltage, this would require the battery to convert from DC to AC and then the charger to convert from AC to DC. Even if this were available on the market (for external usage purposes) they would be bulky, inefficient and pointless for your use.
So, keeping that in mind what you want is between 14.5 VDC to 18.5 VDC (Magsafe charger Wikipedia),
Alright, now let's look at your options,
The Expert Battery Geeks seem to have a webpage with a list of some more, but I would personally stick with one of the above products.
Now, unfortunately any of these products don't seem to come with a MagSafe cable (thanks to Apple they are not allowed to manufacture it because Apple refuses to license the connector), but these should be fairly easy to find on eBay or other websites depending on what specific device you want to charge and its possible that it's bundled with one of the devices I mentioned.
Albeit insanely expensive and most probably less efficient than the other options I mentioned, this HyperJuice pack offers support with the Mac. They basically buy a Mac charger from Apple for you and create the cable you require. It seems although that this may not solve the problem with your camera though because it doesn't support charging at different voltages, so you risk spoiling your camera. The other ones mentioned on top will not have that problem.
I saw on TV that it exists a solar charger around 17V. I guess that this could be an easy answer to your troubles. I've never tried it myself, but this is quite interesting.
And as Aditya said, I think that anyway, you will have to buy a MagSafe cable (Apple doing Apple stuff :) )
THIS WON'T WORK
The approach I would recommend in this case is, for each device you want to charge, get a charger suitable for use in a car (ie from a 12V DC socket, like the cigarette lighter socket in cars) and then get a large 12V battery with that type of outlet.
The batteries are generally sold for use with CPAP devices (a medical device used to treat sleep apnoea).
An example of such a battery is here: http://www.amazon.com/Battery-available-C-100-1-Batteries-C-100-1-SR/dp/B00AJPW1DK/ though I would check that this includes the cigarette lighter socket. I've used similar batteries when on holiday with my CPAP.
This would mean buying a car-charge adaptor for your camera and for your MacBook.