I have an outdoor timer with two prongs (ie no ground). The wall outlet itself has GFCI. I have a three pronged (ie with ground) 3-outlet adaptor that does not fit into the timer as the ground prong hits up against the plastic/rubber enclosure of the timer. See photo attached (at top is the timer to go into the GFCI outlet, middle is the adaptor to go into the timer and at bottom the two-pronged extension cord). How can I string two or three extension cords (with either three prongs or two prongs) from the timer using the adaptor? I have not been able to find similar adaptors but with two prongs (ie without the ground) at my local stores.
Two-prong splitters are not hard to find. Expand your search to big box stores or online retailers.
You can also use the ubiquitous 3-to-2 adapter (cheater plug):
This adapter is safe to use in this case because of the GFCI outlet.
However, if you're in a pinch, just break off the ground prong on that green splitter and plug the ground holes on the outlets with caulk or something else non-conductive.
Don't use it for anything requiring equipment grounding. You're responsible for acting like an adult and not abusing this equipment (or a cheater plug, or whatever) or allowing less clever people to do so as a result.
With a little DIY you can create a safe alternative than a cheater plug.
Find a box so you can mount the timer box and a few sockets into the side with the controls accessible without opening the box.
Find a cord with attached 3 prong plug. And one of more 3 prong sockets.
Connect the ground from the cord tot he ground of the sockets.
Connect live and neutral of the cord to the input of the timer.
Connect the output of the timer to the live and neutral of the socket.
The latter two steps may be easier if you cut the connection from the timer's plug to the timer and splice in there. Otherwise you will need to find a 2 prong plug and socket to make the connection.
Essentially you make the ground connection bypass the timer and run the power through the timer.
You can also add a relay to switch the live connection if the timer is on should you find that the power you need to switch exceeds what the timer can handle.