What diameter pipe should I run from a septic pump up hill (3 meter head ) to my septic tank?
The pump and pipe (force main, being on the far side of the pump) size should be specified by your septic system engineering documents. While I feel similar to the commentators about pumping sewage, sometimes it's the only way (house on a lake, for instance.) 

I would strongly suggest both the "two pump and alarm" method (preferably with a pump controller that uses both pumps in an alternating fashion - having a "spare" sit there while the main pump works tends to lead to having two pumps that don't work when the main one breaks) and a mounting method that permits the pump to be pulled out for service (typically the pump is on rails, and the system automatically valves off the pipe when the pump is pulled out.) They are disgusting and expensive enough to repair without setting them up in a manner that requires a pump out and someone going into the hole to fix them.