If I install 3 Linux OS on a 128 GB USB, will it boot with options on another computer?
 

If I create a USB stick with three Linux distros on it, when I plug it in to another computer and boot from it, will there be the three options to boot from at start up? Or just, it boots from one, with no options...?

if you are talking about a full install to the usb, as long as you also install a bootloader (which is pretty much standard with all distros) you should have a choice. the same should apply if you are using a tool like ventoy or yumi.

it might help to know how you plan on creating your usb.

These days, it will need to boot from both MBR & UEFI, but otherwise, it should do, unless you have some unusual hardware.