Installing RAM in pairs. What about 3 gb?

Hi. We all know that ram has to be installed in pairs… How does that work for 3 gb? Will a pair of 2gb and 1 gb work or can we install 3 separate 1 gb sticks?

i got a lenovo core2duo pc with Intel inside.

Added (1). i have 4 RAM slots.

Depends on how many RAM slots you have.

"we all know that ram has to be installed in pairs…"

No it doesn't…

Only Dual Channel RAM should be installed in pairs. That is for a motherboard that has that support. You will see RAM slot plastic in coordinated colors. Install same size strips into same color slots.
You can put two 1GB strips into the black slots (for example), and two 512MB strips into the gray slots (for example). Then you have 3GB in Dual Channel RAM configuration.
If your MB has slot plastic of all one color and has no user guide instruction about Dual Channel RAM support, then it does not matter if you put 3 pieces of 1GB each into 3 slots or one 2GB and one 1GB strip into 2 slots

(2 x 1Gb) +( 2 x 512mb) = 3 Gb