I want to charge 3 separate 18650 cells with one charger. Each one of the batteries has their own TP4056 module. So in total there are 3 TP4056's. I know that TP4056 is rated at 1 amp input, though my USB charger only puts out 2 amps. I want to use USB male to 3 Micro USB splitter, to charge each battery equally, and by doing calculation: 2/3=0.666, each battery only gets 0.666 amps. So is 0.666A enough for TP4056 to charge a 18650 battery?
From the TP4056 datasheet:
Programmable Charge Current Up to 1000mA
That means the TP4056 is able to handle charge currents from 0 to 1A (1000 mA). If you're only able to supply it with 2/3A (~666mA), so be it. All that happens is that it will take longer time to charge the cell/s.
Edit: As pointed out by Bimpelrekkie, you have to make sure that the USB charger you have in mind for this application behaves properly when subjected to this heavy current draw. But still, the answer to your question "is 0.666A enough for TP4056 to charge a 18650 battery?" is Yes, you can charge a 18650 cell using less than 1A of supply current.