Is it possible that the ultrasound technician could not see a triplet hiding behind twins at 11 weeks 3 days?

Yes: Depending on how thorough the study & the training of the tech, missing a fetus is possible. Regular US is a 2 dimensional study and you only see what your probe lights up.

Possible: Depending on the expertise and experience of radiographer plus based on amount of technical difficulties arising during examination , it is possible to miss .