I can see why Galahad was not in the game., and why Ciri and other story elements were changed in general. Sure one reason is the convenience, but I believe there is another reason.
IMO, the books and games have quite a different tone. The books are like an amalgamation of various Slavic and Anglo-Saxon myths/fairy tales, with a very "self aware" or "tongue in cheek" look at classic conventions within the story in a very realistic manner, thus there is a sense of absurdity, surrealism and silliness that the books allow from time to time that the games do not.
The games on the other hand almost take themselves too seriously for the lack of a better word, so having a character from Arthurian legend may seem jarring to say the least. If I didn't read the books, having Galahad appear in TW3 would be like King Henry appearing in Game of Thrones. Not to say that TW3 doesn't have its strange/funny moments, but its not metaphysical like the books.
For what its worth though, I thought the first game captured the feeling and tone of the books very well, and if CDPR continued with that tone throughout the trilogy, than a Galahad appearance in the games wouldn't have seemed so strange. But ever since the second game, it seems CDPR is trying to emulate "Game of thrones" or "A Song of Ice and Fire" tone/vibe rather than the very source material it is based on.