Following this tutorial on "Understanding ROS Services and Parameters" (not allowed to add a link) (12.04, hydro)

Question: how can one use rosparam to affect more than one turtlesim? The lines below seem to affect only the one that was created last. What if I started three from different Terminals and wanted to change all the backgrounds in one action?

$ rosparam set background_r 150
$ rosservice call clear

Thank you