How to generate more current than available DC
There is but one absolute rule in life when it comes to power supplies...

"You can not get more energy out of a circuit than you put in."

As such your supply is 5V at 0.9A, that is 4.5 Watt.seconds. You are asking for 5V at 3A, that is 15.s. There is no way to do that continually without adding more power from somewhere else. The physic's police will arrest you if you do.

However, if your load is not continuous, you can output 15W.s for 4.5/15 (0.3) amount of the time.  Actually depending on the efficiency of however you are storing that energy, and how much quiescent current the rest of your circuit uses, less than that, perhaps a fifth or a quarter of the time, or less. Storage options include but are not limited to, large capacitors, chargeable batteries etc.

Whether whatever you are doing has the time windows available to store that amount of energy efficiently is beyond the information given in your question.