The art style had a "ligne clare" realistic style, Franco-Belgian, similar to Jacques Martin's Alix. The story was set in the Arabian or Sahara desert. One of the boys was blond and fair-skinned. The other two were brown skinned and looked Arabian or Indian. One of them was a girl.
The antagonist was a snake man with a cobra head and magical or telepathic powers. In the final moments of the adventure, the blond boy, the Arab girl, and a wise old man flee aboard an improvised raft through an underground river and discover that they are going to fall through a very high waterfall. The wise old man takes all their clothes off and, with the fabric and the sticks from the raft, makes a parachute in a hurry.
I remember very well the images of the naked girl in those last pages. I was very impressed, I didn't know why.
It was in a Spanish magazine published by Bruguera, maybe Mortadelo or Zipi y Zape. It was part of a series, from the 1960s/70s, and was Franco-Belgian.