How to boost a Power Bank's maximum output voltage?
All the 5 V outputs share the same ground hence they cannot be connected in series. To make it easier to understand you can think that the outputs are already in a parallel configuration and hence cannot be changed. What you can do however is use a boost converter, which increases/boost your voltage to a certain extent (depending on the IC used). To clarify, you can connect it to one of the 5 V outputs and increase voltage. The drawback is that the max current at the output will be capped (limited to an extent). 

Further clarification is required on the use of AH units, AH is a measure of battery capacity. For example 3 AH means that by supplying 3 amp continuously; the battery can survive for an hour. However, this is a theoretical value as voltage decreases as the battery drains out. if the voltage decreases then so does the current when used on a resistive load. Anyways, your battery can supply more current than 3 A but it will last for less time.