howto create Multi Target with cacti ?

[cc]

hi

How can I create Multi Target in Cacti or rrdtool and show only as one graph ?
I sniff with cacti: 2 workstations and 1 printer : everything individually.
My problem is:
I want to collect (add) 3 target as a sum and show them as only one target
with only one graph.

greetings
cc