Hi,
I don't think it is possible.
uboot can be used to setup a multi-boot system but all have to use the same kernel. since beta3 uses a 2.4 kernel and r198 a 2.6 kernel you could say you have a problem.
To use multiboot :
uboot uses some predefined keys to select a drive or system to boot from, just press the key while booting :
"OK" : to flash a new kernel/image
"1" : boot from /dev/mmcblk0p1 (sd)
"2" : boot from /dev/hda1 (microdrive)
"3" : boot from /dev/hdc1 (cf)
"4" : don't remember
nothing : boot from flash
I'm not really sure about the exact order or functions of 1, 2, 3, 4 - just try and you will see (I'm using "nothing" and "2").
Chero.