When he was first captured by Spavid in Robot Reboot, he was able to escape with relative ease. As soon as he did, he was able to gain access to Galaxy High 's Mainframe Computer . It was by chance that Josh accessed the computer to download an upgrade for Fluffy , to which he also downloaded Rex-3 with it. From there, he was able to control Fluffy and eventually all the other ultrapets on campus. It wasn't until Brett launched a digital capsule that the ultrapets were freed from Rex's control, including Fluffy 's. He was then captured by Yoko and then brought back to Galaxy High for containment.
He escaped once again in Toy Galaxy, and this time he had other plans. When he overheard Principal Kirkpatrick speak about Marty McBoogle and the upcoming Team Galaxy toy line, he hatches a plan to override the toys and design them to become evil and attack the marshals. For the most part, he was successful in his plans, until Josh outsmarts them and causes all their ships to collide with each other, destroying them and Rex-3's hopes and dreams of world domination.
- Rex-3 could easily be compared to the character Plankton from Spongebob Squarepants, as the two share some similarites, both in size and motive.
- In the first season, Rex-3 had a high pitch, almost chipmunk voice. In season two, he had a lower, raspier voice. Reasons for this change remain unknown.
- Rex-3 is one of the smallest villains in the series, along with the aliens in Yoko's Secret and Class of 2051.