How can I make an ammeter for ~5V DC up to 3A? I want to connect this ammeter to a microcontroller.

I want to connect this ammeter to a microcontroller ADC and display it on 7-segment displays. I guess a 10W resistor ~0.1ohms or something will be involved. I can obtain them easily. I want to make an inline USB ammeter. I am experienced with PICAXE microcontrollers and surface-mount.

I have not actually tried this little gizmo, but I saw this link on eBay, from one of the Hong Kong mongers:
http://www.ebay.com/itm/ACS712-Module-20A-Range-Hall-Current-Sensor-Module-/150851247617
It is a Hall effect based current sensor based on an ACS712 integrated circuit, made by Alegro(r).  Data sheet here:
http://www.allegromicro.com/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs/~/media/Files/Datasheets/ACS712-Datasheet.ashx

It kind of looks like, you give it  5 VDC, and then it gives you an analog Vout = 2.5 V + (100mV/ A) *I, where I current (in amperes) flowing through that little shunt inside the IC, and if I can believe what the datasheet says, that shunt itself has a resistance of only 1.2 milliohm = 0.0012 ohm.
NICE chip. I have some projects to use that one in.
On 5V supplies, its a bad idea to use a resistor as big as that, because the forward voltage drop will cause your 5V circuit to malfunction at full load, when the system will only see 0.5V.

MUCH better would be to use an even smaller resistor, made of a piece of resistance wire, and put an amplifier to amplify the differential across it.

THE BEST way I know of, is to use a device made by Zetek (allthough folks like Linear technology do similar), the ZXCT 1082 series, which can use a piece of PCB track as the current shunt ( though I don't like that approach much)