HLDS is single-threaded, So each server will use only one core, in your case there are 4, You can run 3 hlds instances easily, and you have enough memory so that's not a concern, what is left is your upload speed which is an important factor on how many players your machine can handle with a stable ping, now your upload speed is enough for 36 slots in total ( 12 x 3 ), you can calculate the rates for a smooth experience here:
https://forums.alliedmods.net/showthread.php?t=112824