BTC stands for Bitcoin, and it is the actual currency. mBTC and uBTC are simply different ways to view it.
mBTC is a thousandth of a Bitcoin, whilst uBTC is a millionth of a Bitcoin.
BTC = 1 Whole Bitcoin
mBTC = 0.001 BTC
uBTC = 0.000001 BTC
BTC = 1 Whole Bitcoin
mBTC = 0.001 BTC
uBTC = 0.000001 BTC