A320 Partial Ground Spoiler deployment
 

One for our Airbus friends...

On a landing roll today, as a passenger behind the wing, only 3 out of 6 Ground Spoilers were used on landing. I also noticed this during the Flight Control check out the window at the departure airport, only 3 were raised. Full flight, short runways used also.

Never seen it before after many hundred Airbus flights, why is this the case?

wheels down, the image in the link shows the system architecture and may help you identify which ones were not working.
http://hsto.org/storage2/4a3/d1a/78e...95edd7d29a.jpg

On a tangent, to remember your hydraulic source for spoilers, left to right:

GoodYear Brings You Grip

In OP case, it would most likely be a SEC computer not working and accepted to operate under the MEL.

Thanks guys
Here is the photo...

http://i62.tinypic.com/7137fl.jpg

Therefore from that photo, SEC3 isn't working.

Would you notice any significant effect with the reduced spoliers on rollout? Not an Airbus pilot, but one would assume longer/heavier braking required?

Longer certainly. Tables in the QRH give you landing distances with failures of various systems.

The main function of the ground spoilers is to kill the lift of the wing, to put the aircraft weight onto the wheels to ensure good tyre adhesion and wheel brake performance.

I can see spoilers 1+2 down, and 3,4,5 raised?

Link to an A320 wing pic on roll-out.

Photos: Airbus A320-233 Aircraft Pictures | Airliners.net