The entire philosophy behind the Airbus movement in the 1970s was geared towards a widebody twin and the cost savings it signified during the oil crises of that time. The A300 was a huge breakthrough in that context. Similarly, the A310 was the first aircraft to gain ETOPS approval as a longhaul widebody. The only reason Airbus even has the A340 is to be able to play the ETOPS 207 card against the Boeing 777. If you look at the entire Airbus line, their only product that is below contemporary engineering standards is the A340 - their only non-twinjet product. Any Airbus attempt to develop a trijet would go counter to the bedrock principles they have built their engineering on, namely the design that makes the most sense from an economic (and now environmental) standpoint.
Additionally, with the advent of ETOPS and LROPS, the need for a trijet has now disappeared. You don't need a third engine for safety purposes and you don't need it for the thrust. So why throw in an extra variable, when the equation has already been solved for you?
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada