I know that there was at least some Celtic influence in Poland, but looking through various online resources for Polish and Slavic mythology, i couldn't find any deitiy that sounds quite like the goddess Melitele, with her three aspects: maiden, mother, matriarch.

Are there any specific Slavic deities that Sapowski/CDPR is drawing on, or is this goddess imported directly from conventional Celtic lore?

Modern, not historic. See Robert Graves, The White Goddess. Graves was not concerned with historicity, and there is no solid evidence for his claims, which are pretty much uniformly rejected by legitimate anthropologists. There is no "conventional Celtic lore" that Sapkowski could have drawn on, only what Graves claimed was such.

No matter what his scholarship, Graves was enormously influential, and the Triple Goddess (Maiden, Mother, Crone) is widespread in fantasy.

There was maybe something before accepting Christianity 1000 years ago when all Slavs have had many pagan gods, but I don't know exactly.

While there are some depictions of Hekate as three-faced, she is not associated with the three ages of the Triple Goddess; she is a perpetual maiden in every well-founded account.

Not among the major deities, anyway. IIRC, the most significant goddess, Morana, forms a dualism with her brother Jarilo. (Two of the principal gods, Perun and Svarog, do appear in the names of runestones in TW1.)

There is much we will never know about historical European paganism, though, because most of it was passed down by oral tradition, and chroniclers such as Saxo Grammaticus wrote with a frank Christian bias.

May be you should first read the article I linked before giving an ignorant comment?
Check the 5th century BCE sculpture, there are clearly a child and an old woman as a different representations of Hekate (you cannot see the third figure clearly).
Citations:
1. "She has been associated with childbirth, nurturing the young, gates and walls, doorways, crossroads, magic, lunar lore, torches and dogs."
2. "Greeks held the yew to be sacred to Hecate, queen of the underworld, crone aspect of the Triple Goddess."
3. "As a virgin goddess, she remained unmarried and had no regular consort, though some traditions named her as the mother of Scylla"

Even this is enough to make a lot of parallels with Melitele.

I did read the article; I just believe it is unsound. It is dubious scholarship, of which there is much. Hecate is not the Triple Goddess of the Neopagan movement; claims that she is ever portrayed in three ages are not substantiated, and even the three-faced portrayals are Hellenistic.

You don't believe the scientific method? Ah, OK then, cannot argue with that. lol

In any case, the question was where the Melitele is coming from. The answer is that she was modeled after a description of Hekate made by some people. And it's irrelevant if was described right or wrong by those people relative to original real Hekate cult. It's taken directly from that description, period.
Here is another link:
http://www.moonlitriver.com/hecate2_gallery.html
Read the first paragraph and try to prove that this is not the Witcher's Melitele almost word to word.

I don't dispute for a moment that there are modern constructions that are identical with Melitele. The articles you cited describe them clearly.

What I dispute is only that there is any well-founded evidence that any such goddess was worshipped anciently. The earliest representations of Hecate as a triple figure are Hellenistic: 3rd C. BCE, and clearly due to syncretism. And then they are not of the "three ages". There is a well-sourced account of these at the Theoi Project: http://www.theoi.com/Khthonios/Hekate.html

The articles you cited, most notably the latter, are mere assertions; there is nothing of scientific method or valid anthropology about them.

The worship of the so-called "Triple Goddess" is, rather, a modern invention and largely attributable to the work of Robert Graves. No such claim precedes Graves' authorship, and no claim that is not derivative of it is made.