I've been trying to remember the title of a children's or young adult book I read as a child about thirty years ago. (So it may be from '70s or '80s). I don't remember many of the details.
There were three main protagonists; I want to say that all three were boys (maybe one was a girl), one of them may have been from India and the other two from the US or UK. The three end up in a fantasy land with an evil king or prince.
There's a giant "inchworm" that the three characters can ride on saddles, which means that the middle rider goes up and down as the worm moves. As the worm is running from guards at one point, the middle rider gets jammed up into a crack in the ceiling of a tunnel and gets separated from the other two.
I also vaguely remember a scene where a character eats some glowing mushrooms and ends up glowing himself, being mistaken for a ghost.
Odd memories, but I do remember enjoying the book as a child and would like to read it to my own son. I've made multiple searches online trying to word the search in every way I can think of without success.