PPVBUS_USB (L5700) should not be dropping down to 3V when charging (or even 4V for that matter). To things could be going on here:
# The power source (charging brick or USB port) can't supply the necessary current and is being current limited , hence dropping the voltage.
# The power source is fine but the device is drawing excessive current due to a short or partial short somewhere.
If you have access to a [https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=USB+Ammeter|USB Ammeter], you could use that to determine if the device is really drawing current when it says it's charging.