The Collaborative International Dictionary of English v.0.48:

Ter- \Ter-\
   A combining form from L. ter signifying three times, thrice.
   See Tri-, 2.
   [1913 Webster]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

TER
       Typo3 Extension Repository (Typo3)