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This is a Fantasy fiction/Science Fiction book I read that my dad owned, was probably back in the mid 90's that I read it.

Two kids (I think they were brother and sister) were kidnapped and transported to another world, a world of magic (Medieval sort of setting, very war-like). There were also other people who could travel from the world of Magic, this planet, I don't remember the name of, and Earth, the planet of technology. The goal of the Magic planet was to open the gate to the third planet, which used both technology AND magic. I think I remember the magic world preparing for an invasion of Earth as well in order to gain access to this new third world. I don't recall it being religion-based, I think it was very science-based. The magicians I believe used magic as a form of science or something. Details are very vague.

I think the one who kidnapped the kids was from the Magic world, or maybe it was a mistake? I kinda remember there were 2 "parties" that the view followed as well.

I remember the kids having magical abilities, once they got to the magic planet at least. There were 3 types of magic, each one harder than the last. I don't remember the first two, probably like visualizing, then transforming, but the last was creation, which you created something out of nothing. The boy was good at doing that, and bad at the others. The adult person who was with them realized what he was doing and was stunned, because that's the difficult magic to use. He was pretty much making magic air bullets for their gun, and just kept reloading it.

Can anyone remember this series? I think it was a trilogy, but this was some 20 years or so ago that I read it.

This description is super familiar! I'm wracking my brain and the best I can come up with is the Warriors Trilogy. The siblings are named Dan and Ursula. I recall there being time travel involved. Ursula spends a good amount of time in disguise as a young man, but at one point accidentally changes her appearance through magic.

I think that the siblings end up in a different time period during each book, and at least one of them is magical/medieval. I think it ties into the Arthur legend somehow.

The Doomspell trilogy by Cliff McNish? it sounds pretty similar to your description.

Summary of the first book:

n The Doomspell, Rachel and, Eric are sucked into a freezing world called Itrea, by a witch called Dragwena. They meet a nice old man named Morpeth, who is over 500 years old and, like Rachel and Eric, was removed from Earth to someone by Dragwena. Rachel is fooled into believing that she is becoming a witch by Dragwena, and her Magic, trying to be helpful, tries to make her a witch. During the transformation, Dragwena lets Rachel inside her mind, giving Rachel access to the spells there. As Rachel's skills develop, she finds that she is more than a match for Dragwena. With the help of the Wizard Larpskendya, she kills Dragwena, and Larpskendya removes the spells making it snow. Morpeth then returns to Earth with Rachel and Eric.

I am not sure. There are inconsistencies, but it does sound similar to "The Secret of Platform 13", where the there is an isolated island that's entrance to the normal world only opens every few years and is located under an abandoned train platform, and the King and Queen's baby is lost out in the normal world and they have to wait like 10 years and then come back to recieve the now, like, 10-year-old kid.