Apple only used 5 and 10MB scsi built in that I know. Technically 5 is SCSI1 and 10 would be SCSI2...
[]NITPICK] That's not really true and has caused no end of confusion. SCSI 1 and SCSI 2 are revisions of the SCSI standard, although it's arguable that SCSI I never existed or was never codified.
Unenhanced SCSI 1 and SCSI 2 are both capable of 5 MB/s performance, in theory. The SCSI 2 standard offered (optional) enhancements that allows SCSI to operate at 10 MB/s (Fast or Wide) or 20 MB/s (Fast and Wide both together). In the wild you never see Wide without Fast, but according to the Standard, it could exist.
So, anyway, to say something is SCSI 2 doesn't really tell you anything about the data rate other than that it is 20 MB/s or less. If could be 5, it could be 10. [/NITPICK]
On Apple's implementations, if there was an external SCSI port, the speed was always 5 MB/s. I suppose I should nitpick myself here, as that's only true for external SCSI ports built into machines. If the port was on a card, Apple offered faster SCSI options on some models, then it might be faster. But it is also the case that most of the time when Apple offered an installed faster SCSI card option, they did everythign they could to block off (or omit entirely) the external SCSI port on the card.
According to the Apple Developer Notes the PowerBook G3 (depending on model) had either a Heathrow I/O Controller or a O'Hare I/O Controller. The SCSI port comes from the hardware in the I/O controller. Beige G3 computers had Heathrow. PowerMac 5500/6500 had O'Hare.
In either case, the SCSI port(s) only supported 5 MB/S.