You would first need to get Fuse working with the kernel. If/once we can boot custom kernels, it's possible one might ship with Fuse enabled but according to
http://groups.google.com/group/android-kernel/browse_thread/thread/096dda136ce772cb
stock does not have Fuse. Stock does support modules though and I'm fairly sure you can compile Fuse as a module. You'd need to get ahold of the Droid 3 stock kernel source, cross compile a module for Arm and try and get it to not kernel panic the phone. Once you have Fuse in there, it shouldn't be too bad to build a copy of sshfs.
In the mean time, I have found
https://market.android.com/details?id=nextapp.fx to be an incredible file manager that support SSH, SMB, and FTP. The only thing missing would be root.
Anyway, I think Fuse support would be awesome but don't have time to work on it at the moment. Do we have the D3 kernel source somewhere?