hi from me as well! Yes I am a greek witch, but my practice is not based exactly in Wicca and neo paganism but the understanding I have from ancient greek literature. The way that I understand triplicity is completely different than someone elses who practices Nordic witchcraft or is a Wiccan.
For example with Hecate, for me, there is an underlying sense of triplicity of her (the goddess herself, not a grup of three), that actually came later into her worship perhaps during the Hellenistic period, where there was the first appearence of a type of reliefs and statues of her called hekataion. The basic form of the Hekataion was a pole having the form of Hecate around it three times. It was usually put outside of temples or crossroads of three ways, each body facing a road so that she can watch over the path. Later in time and into the roman period the three bodies merged and became one body with three heads, again wacthing over path ways.
Another maybe read to the triple nature of her, it could be her association to the moon, so, each of the bodies/faces represent phases of the moon.
Or according to Hesiodus and Theogony, because of her ancestry is the goddess that rules three Kingdoms: Heaven, Land and Sea, ruling from the time of the Titans until Zeus became King of the gods. But because she helped the Olympians win at the Titanomachy, honoured her by letting her keep rulership over part of the realms that previously belonged to her.
Actually one of her epithets is Trimorphe (Τρίμορφη), whict means: three-formed.
I don’t know where this comes from, maybe is a wiccan thing I am not aware about, I would not put those three toghether. IF Hecate was part of a three group that would be Persephone, Demeter, Hecate. Hecate after all, before she got her “underwordly crown” was a titaness who helped Demeter in the search of Persephone in the underworld.
Perhaps the Artemis part might come from the roman times where they knew her with her epithet Trimorphe, in Latin Trivia or Diva Triformis where for them it was Diana (a goddess that had aspects from both Artemis and Hecate)
There are so much more to unpack regarding the gods, especially the Greek-Roman ones because of the shared history of the two civilizations some stories become so perplexed and you get lost.
Anyway, my advice would be, unless you are following wicca or any other religion that has rules, to go with your gut and what feels better for you. My go-to-pair of dieties if I need a feminine and a masculine energy are Aphrodite and Ares.
Also you can worship as many dieties as you like, but as previously said (sorry I don;t remember which witch said it 
) go slowly and try to have dieties that like or are ok with each other. Mine are Aphrodite, Ares, Hestia and now I am getting to know Seshat, an egyptian one.