Understanding Buck Converters and Solar Panels
You need to understand that input and output power from buck converter are the same, minus converter losses. Say on input you have 10 V and 10 A, with 50% duty cycle you will get on the output 5 V and 20 A. Now you say that you want to go from 5 V to 3 V, which is 60% duty cycle, in which case current goes from 80 mA to 130 mA.
Can you provide a datasheet for that PV panel? And please describe your application, there are many possible ways to get what you need. For example you could avoid buck converter by connecting a 3-V battery directly to the PV panel, in which case you will get surge current capability and voltage you need.