USB to 3-cell Lipo charger?
As options for three-cell Li-ion/LiPo charging, you may choose from


BQ24105, standalone version (datasheet)
BQ24115, system-controlled variant (datasheet)


Alternatively, you may use LTC4007 (datasheet), however, this requires an input voltage of at least 6V, so you'll have to supply an in-between boost mechanism from the USB voltage of 5V.