In theory, images built for the 3B should work fine on the 3B+ - the SoC
is the same (but with a clock speed boost). You have a choice between
building aarch32 and aarch64 images - make sure your config.txt is
configured such that it will boot the kernel in aarch64 mode if you
decide to boot an aarch64 image ('arm_control=0x200' etc.)
There are some differences between the 3B and 3B+ as far as other
hardware on the board goes (i.e wi-fi & usb/eth chip), but we don't
provide support for that.