FanDraft doesn't directly support 3rd Round Reversal Draft Orders. However, you can still use FanDraft, by following and understanding the instructions and information below:

You must first set up the official order and denote it as Serpentine or Non-Serpentine. You will then you'll have to adjust each individual pick to adjust for each round to create the reversal.  

It is important to understand how the draft board works, and how this affects your 3rd Round Reverse Order, by reading and comprehending the further information below:

The Draft Board Display and How it Works

FanDraft works in the same vein as the traditional "Paper/Cardboard" draft board concept. Teams are aligned from right to left based on the official draft order. Players are then placed from left-to-right in the order they are selected.

 If a league's Draft Order is set up as "Serpentine", it will reverse from right-to-left in alternating rounds.

This board display is ideal for allowing owners to see what player was taken at what pick during the draft.
If you have an unordinary draft order (switches between serpentine & non-serpentine throughout the draft), you may find it best to use Roster Board Display during the duration of your draft, as the traditional "Draft Board" display view becomes irrelevant in these non-conventional draft orders.

The Roster Board Display can be viewed/sorted by two methods, "by Position" or "by Pick".
You can change which view in Draft > Draft Options, and changing the related slider.