Ford Cmax faulty fuel injectors

In November 2012 we purchased a 2009 Ford C Max Zetec Diesel 1.8 Tdci with about 19,000 miles on the clock from a branch of a large motor sales chain. The car now has 45,000 miles on the clock and full service history. In the last three years we have owned it we have had three fuel injector failures. The specific questions we would like answered are:

(1) is it normal for a Ford C Max of this age and mileage to have three fuel injector failures over a three year period?
(2) what are the likely causes?

Moe details are provided below:
The first fuel injector failure occurred in late 2012 three weeks after we bought the car when the car unexpectedly went into limp mode. The seller repaired the car by replacing one of the fuel injectors. In October 2014 the car broke down again during a holiday (limp mode). The garage found a fault code relating to the number 3 injector which they replaced. In August and September this year the car again broke down (limp mode). A different garage has diagnosed fault codes for number 4 injector, which needs to be replaced. We have always filled the car with diesel at reputable garages (Shell,Tesco etc) and have had it serviced at a large, reputable garage chain.