Say, I just had an idea:

From the time the rats are very young, take three females and toss them gently from one hand to the other.  Give them a delicious treat each time.  They eventually come to adore being tossed and anxiously look forward to it.

Over time, toss them further and further apart, until you're tossing them with your arms open fairly wide.  One at a time…a gentle lob and o-o-o-o-over she goes, caught safely in the other hand.

During this time, teach them to run up the hand that's caught them, across your shoulders and down the other arm, ready to be tossed again and receive another delicious treat!

Now toss all three rats, one after the other.  They're caught and immediately scamper back as fast as they can for another toss.

Rat juggling.