you can have AmigaOS3 9, AmigaOS4 and MOS for PowerUP on the same disk.
The only advice, since the 3 OSs installs some resident stuff, it would be better to switchOFF the system switching from an OS to the other (there are "tricks" to avoid this).
i.e If you give to AmigaOS4 the higher priority, to be able to boot in AmigaOS3.9, you have to enter in the EarlySS, chose the OS3.9 boot partition, disable the AOS4 partition doing all this 2 times (because AOS3.9 perform a second reboot at the setpatch)