I think it's unlikely that a Pi would work well, if at all, with FreeNAS.

Note the recommendation on their download page:

The new recommended boot device size is 8GB. FreeNAS 11 requires 8GB of RAM to run properly.

The Raspberry Pi 3 has 1GB of RAM. So that requirement seems to not be met, and it's not looking great so far for FreeNAS.

obarthelemy of the Raspberry Pi forums also suggests that because FreeNAS is built on a BSD variant, that is also likely to be problematic:

Yes, you can use the Pi as a NAS server.

But not FreeNAS per se: FreeNAS is built on a flavour of BSD, not Linux, and as far as I know, no ports of any *BSD in general, nor FreeNAS in particular, are planned.

So you'll have to build your NAS starting from scratch, from a regular Linux distro which is a little more work. Also, the Pi does not have very good I/O (everything goes through a single USB2 port, even on the model B), so don't expect high performance nor high capacity. Anything beyond 1 disk (USB2 only) and 2-3 users will be very iffy.

Note that this was written in 2011 for the original Pi, not for the Pi 3. Support for BSD seems better now in 2017, so whether this is still a concern or not is something to investigate.

MakeUseOf wrote a guide for setting up a NAS using Samba, which might be easier to follow than trying to force your Pi to use FreeNAS when it probably isn't suitable.

If you want to see why using the Pi as a NAS might not be the best idea, you might like to read Raspberry Pi 2 vs. Synology NAS on the DataHoarder subreddit.

I think it's unlikely that a Pi would work well, if at all, with FreeNAS.

Note the recommendation on their download page:

The new recommended boot device size is 8GB. FreeNAS 11 requires 8GB of RAM to run properly.

The Raspberry Pi 3 has 1GB of RAM. So that requirement seems to not be met, and it's not looking great so far for FreeNAS.

obarthelemy of the Raspberry Pi forums also suggests that because FreeNAS is built on a BSD variant, that is also likely to be problematic:

Yes, you can use the Pi as a NAS server.

But not FreeNAS per se: FreeNAS is built on a flavour of BSD, not Linux, and as far as I know, no ports of any *BSD in general, nor FreeNAS in particular, are planned.

So you'll have to build your NAS starting from scratch, from a regular Linux distro which is a little more work. Also, the Pi does not have very good I/O (everything goes through a single USB2 port, even on the model B), so don't expect high performance nor high capacity. Anything beyond 1 disk (USB2 only) and 2-3 users will be very iffy.

Note that this was written in 2011 for the original Pi, not for the Pi 3. Support for BSD seems better now in 2017, so whether this is still a concern or not is something to investigate.

MakeUseOf wrote a guide for setting up a NAS using Samba, which might be easier to follow than trying to force your Pi to use FreeNAS when it probably isn't suitable.

If you want to see why using the Pi as a NAS might not be the best idea, you might like to read Raspberry Pi 2 vs. Synology NAS on the DataHoarder subreddit.