By the aft forecastle deck I presume you mean the Poop Deck.
On the port side of the 3rd Class Stairway are 2nd and then 3rd Class men's lavatories. On the starboard side the same for the ladies. 2nd Class also having 3/4 bathrooms per side. Directly aft of the 3rd Class stairs were two 3rd Class bathrooms, with a number of cabins aft of that. You can see some of the details in the plans on this web-site. Going forward the 3rd Class Waiting Room and Surgery were located between the Stairs and the No 6 Hatch.
With reference to the D-deck plan here at ET, the small triangular area at the bow is a store, then working backwards you have the firemens' quarters, the firemen's washroom (port) and toilets (starboard) on either side of the spiral stairs, then the large 3rd Class open space around No 2 and 3 hatches.
From what I gather from my own research into the subject, the Third Class Forecastle General Area was very similar in decor to the Third Class General Room below the Poop, plain white paneled walls, long benches and maybe a few tables and chairs. I can only speculate that the floor tiles were the same as those used in aft General Room.
My own impression of the area was convenience for the passengers booked up near the bow so they would not have to walk the length of the ship to get to a lounge area.
In the Shipbuilder under the general heading: Promenade Space: ...."The third-class passengers have a promenade on the shelter deck in the after well, and a large covered-in space on D deck forwrad, which is fitted with tables and seats, and can be used in any kind of weather".
There's a small photo in Ghosts of the Abyss (page 116) which shows the remains of a couple of those tables in the wreck. Only the cast iron bases (still fixed to the deck) and the metal frames of the table tops have survived. They appear to be of the refectory type, similar to those used in the 3rd Class dining room.