Can I use it for pi3 b+?

[qwerlkjasdf]

Want to by pi3 b+, is it will be ok with MACHINE="raspberrypi3"?

[OldManYellsAtCloud]

I'd rather try MACHINE="raspberrypi3-64" for 64-bit support, but yes, it should work: the 3b+ devicetree is installed, and supposed to be used automatically upon boot.

[bhargavdas]

Want to by pi3 b+, is it will be ok with MACHINE="raspberrypi3"?

Yes you can both, MACHINE = "raspberrypi3" and MACHINE = "raspberrypi3-64".

To build raspberrypi3, the following config is needed
LICENSE_FLAGS_ACCEPTED = "synaptics-killswitch"

You might be interested in the following configs as well,
ENABLE_UART = "1"
RPI_USE_U_BOOT = "1"