Power from LiPo battery 11.1v- how to best regulate?

I'm building a system of portable devices for paintball/airsoft use, a sort of capture-the-flag gadgets. The code is written, most of the hardware in place and the casing is coming around very nicely. I have an Arduino Mega with an LCD shield, a 3.2" TFT and a DFRobot mp3 player module hooked up to one 3W recycled speaker (see image below). All of this x3, to make 1-2-3 gadgets.

Enter the power stuff. I suck at the electrical side of this.

The best solution by far is to use the same kind of batteries we use when playing, which is the regular RC LiPo batteries with 11,1 V, 11000 mAh. I need it to feed both the Arduino/TFT and the mp3 module (which runs on 3,3V).

What is the best solution for this? Can I hook up the Arduino directly to the battery or will that fry it? I'm a bit pressed for time, so how about just stripping clean a couple of car chargers and using the internals to convert the power for the Arduino, then feed the mp3 module from the Arduino 3,3V output?

Or are there better ways?

Ideally you would use a buck DC-DC converter to reduce the voltage to around 7 volts for the Mega. If the 3.3 Volt current draw is more than what the Mega can comfortably handle use another buck DC-DC to provide the 3.3 directly. The buck DC-DC converters are much more efficient than linear regulators and will let your battery last longer per charge. eBay has lots of them for around a dollar a piece in lots of ten.

Thanks! Yes, I know, the buck regulators would be perfect. Only thing is I don't have time to wait for eBay. Thats why I'm asking about the car chargers, because I have a lot of those lying around.

Then how about Amazon? Sign up for Prime trial membership, order what you need and get it shipped free in two days, one day if available and order is $35 and up.
Keep tabs on the trial expiration and then cancel Prime.

Just do a search on Amazon for DC to DC Converters.

Now you're just beeing anglocentric ;). I live in Sweden, we are basically on the north pole and shipping times for more civilized countries don't apply to us.

Sorry, my bad. St Nick still sleeping?
Get yourself a handful of those DC-DC converters, you can always use them. In the meantime, you could draw current from the balance port, the 4 pin flylead from the battery. There you can tap around 4 Volts, 8 volts, and 12 volts. Plug the Arduino Vin to the 8 volt. Use the 3.3 from the Arduino.
Hopefully you have a good balance charger for the Lipo.

Great advice! Thank you! Didn't know that, at all. A bunch of buck converters already ordered.

Tapping into a single cell from a LiPo stack seems a very bad idea.
It could kill the stack or the charger quickly.

Use a car charger with USB socket, and plug the USB lead from the Mega into it.
That also will also cut current use from your 3-cell LiPo battery in half.
The 3.3volt output from the Mega (~150mA) might not be strong enough for the Mp3 player + speaker.
But I think that module can also run on 5volt.
5volt through USB is limited to 500mA. The Mega uses ~70mA of that.