On my MythTV box, the three big drives are mounted at mount points named /drive1, /drive2, and /drive3.

The mhddfs is mounted at /drives, pooling the other three filesystems into a common mass, with these two commands:

mkdir -p /drives
mhddfs /drive1,/drive2,/drive3 /drives -o \
allow_other,auto_cache,max_write=4194304,uid=1000,gid=1000,mlimit=500G

Most of those mount options aren't really needed, but I want everything on the filesystem to be "owned" by user "mythtv" (uid/gid 1000), and I want to allow larger write() sizes than the piddly default (originally designed for .mp3 files).

A much simplified way, just to try things, would be this:

mkdir -p /drives
mhddfs /drive1,/drive2,/drive3 /drives