I believe you are looking for "Journey Through The Stars" by Clive Endersby. I've been searching for this story for several years and finally had a partial breakthrough on the weekend.
It was a six part story, each story in an anthology series that were read in elementary school, Grade 4/5/6. Typically two stories were read per year, which is possibly why you remember it as a three-part story.
In part one, five children are taken from a Science Museum by a two-headed alien. On the first world they land, they help a group of three-meter tall, blue aliens reclaim their planet from rock-like robots that had been programmed to build space ships, but ended up taking over the planet.
After the children help to shutdown the robots, the blue aliens gift them with supplies and a space ship to try and find their way back to Earth.
In Part 6, the children do return to Earth (via a different technology than the ship provided to them), only to find they were replaced by doppelgangers.
The anthologies series names were, in order:
Grade 4 - Zoom Shots
Grade 4 - Flip Flops (Episode Two: The Silent Planet)
Grade 5 - Ripple Effects
Grade 5 - Time Spinners (Episode Four: The Platorian Zoo)
Grade 6 - Star Flights
Grade 6 - Sky Striders
Episode Two and Four are available via their anthologies on the Internet Archive:
I have so far been unable to find copies of the other four volumes online.