No, no, I meant, use the scripts provided on
this page, but run the second script twice. The first time, using ad0s1 and ad2s1, the second time using ad2s1 and ad4s1. That way, it creates the mirror using the first slice of the first two disks, then it adds the first slice of the third disk. To create the 3-way mirror. That's how I did it at home, and it worked fine.
Basically, the system setup becomes:
do the install to 1 disk, setting up the slices/partitions as you want them to be on all three disks
boot into OS
slice/partition the rest of the disks as needed, using ad0 as the template
run the first script using ad0s1 and ad2s1 for the variable names
reboot using gm0
run the second script using ad0s1 and ad2s1 for the variable names, to sync the two
reboot using gm0
run the second script using ad2s1 and ad4s1 for the variable names, to add the third
At that point, you'll have gm0 made up of ad0s1, ad2s1, and ad4s1, with s3 on all of them available for whatever you want them to be used for.
(Obviously, the ad device numbers will depend on how your devices are attached, I used 0, 2, and 4 just to put numbers in.)