I would rate travelling Economy with those Airlines in order of BA, LH then KLM.
BA aircraft are generally more comfortable, older but reliable entertainment, decent food and usually generous with decent refreshments.
Lufthansa aircraft are newer, but I've found seats on the A340 do get uncomfortable on longer journeys and had IFE problems. Food is reasonable, apart from a spirit before dinner it's really only wine available throughout the flight, if you can find a crew member.
KLM, well last time i flew with them was in 2008 long haul before they changed their 777's to 10 seats across, so the aircraft was older (but comfortable) seats and older entertainment, however the food and drink wasn't very good quality and not much in the way of refreshments.
Obviously these are only my fairly recent experiences. Next BA flight on Friday to MIA.