Question about Raspberry Pi 3 Model B

This can be done, for example, with a BatteryPack:
Xtorm AL450 (12000mAh)

The output should be at least 1.5A (if no additional USB devices are connected) or the official 2.5A.

I have several powerbanks connected to a Y-cable like this one
https://www.startech.com/Cables...
or if that even existed, even a Psi letter(Ψ) shaped "Powerenhancement cable", but unfortunately this cable is nowhere to be found or only at the mentioned shop I don't like, so hop Digitec product manager, please find us something nice at survivable prices XD
{Ach and if you(product manager) should read this right now, it was a damn torture to search for such a cable or other exotics with your filters, there must be more detail, primarily whether you mean socket or plug, as far as I still painfully remember, Microspot has solved it relatively useful, but have in contrast to you again hardly any products in stock, that should not be an advertisement, but only a suggestion for improvement for your immature cable filter system;) I have quite a few orders open right now, you can give me a discount for the golden tip ;D}

Search for "Lithium Battery Expansion Board for Raspberry Pi" or something similar.

If you want to DIY (i.e. put it in a Gameboy case or something), you can get a battery controller from Adafuit, which also controls recharging etc.
Like This: https://www.adafruit.com/product...

e.g. with an additional USB battery, there are many with a 2A output.