So you have three OM3 or OM4 multimode fiber cables?

- Two SC-LC and
- one LC-LC?

Did you properly connect MADI OUT with MADI IN on each of the devices?

           UFX III                        Ferro#1                    Ferro#2                  UFX III
Cable      OUT------------------>IN  OUT--------------->IN  OUT------------->IN
PlugType   SC.........................LC   LC......................LC   LC...................SC

When plugging the simplex cable to the LC port, you need to be careful, to properly plug them in and that the sending port on one device is connected to the receiving port on the next device in the chain and back to the UFX III.

The final locking of the metal latch after inserting the connector can be somewhat more difficult with OM4 multimode simplex cables (possibly also depending on the cable manufacturer), compared to duplex cables.
With duplex variants, the two connected connectors provide more mutual stability, making it easier to lock and unlock the metal latch of the SFP for securing and releasing both, the SFP itself and the plugs of the cables.

When using simplex cables, I have encountered last recently the issue that the locking mechanism can be quite tricky. In my case, the metal latch somehow slipped under the LC connector’s locking mechanism, making it very difficult to release the latch and remove the connector to reinsert it properly.

With my previous duplex cables, the connectors also snapped in more easily and locking the metal latch of the SFP was effortless. Here, I had to slightly bend the plastic latch on the LC plug to get it to fully engage in the SFP and to "snap in".

To rule out any kind of connection errors in this "chain of MADI devices", I would recommend rechecking the entire multimode cabling.

Even though you likely already know this, just to ensure we have the same basic understanding of the device-to-device wiring:

#1 A MADI I/O port on a device has one jack for transmitting and one for receiving.

#2 When daisy-chaining multiple MADI devices, you must connect the transmit port of one device to the receive port
  of the next device, and finally looping back to the recording interface where the chain started.
  So: from UFX III (out) to Ferro#1 (in), from Ferro#1 (out) to Ferro#2 (in), from Ferro#2 (out) back to UFX III (in).

#3 Ensure that the SFP modules are properly seated so that the transceivers are fully inserted and snap into their final position. By locking the metal latch both will be locked, the SFP and the plugs of the multimode cables.

#4 All devices need to be powered (besides proper cabling), otherwise the chain of devices is broken,
      audio transmission towards UFX III won't work and you also do not get clock synchronization.

#5 You checked that the two SFP for the Ferrofish are both for multimode (not singlemode) and have 1310nm wavelength

Also ensuring the following two things might help:

#6 Check if your Ferrofish has a settings which the "preferred clock" is.
      If such a setting is possible, then set it to "MADI optical".
      The M-1620 Pro default setting for preferred clock was "Word Clock" initially.
      When I deleted all MADI routing on the device clock synch failed.
      After setting preferred clock to "MADI optical", then clock synch worked even if the routing of MADI channels
      on the device was not yet done/ready.
      Or in other words, if your Ferro has no setting for preferred clock it might also default to WC.
      Then clock synch possibly also won't work, until you finalized all the MADI routing on each Ferro.
      What needs to be routed? Everything, the channels that you use and those for other devices, see next point.

#7 Ensure that the MADI routing on each of the Ferrofishes is ok and that you route
     - proper MADI in/out channels for the particular device
     - passing through of all other channels (the channels of other devices and those which are not in use)

Please double-check that the multimode cables are correctly connected from device to device (MADI output of one device to the MADI input of the next device in the chain).

Pay particular attention to the proper seating of the SFP and the connectors.
This is easier with the slightly larger SC connector on the UFX III,
whereas potential issues are more likely to occur at the SFP ports of the Ferrofish units.

You can check the clock synchronization status to see, whether the cabling is poper.
Assumed the UFX III is clock master and the ferros are configured as slaves getting clock from their optical MADI inputs.
Best is to ensure that also the MADI frame size is configured to "96k frames" on all devices.

Then you can use the clock synchronization status at two locations
a) MADIface driver settings, you should get for MADI Opt. the status "Sync" (lock is not sufficient).
b) at the front of the UFX III the green Sync Status LED for MADI