You have 3 runs on your transformer to 3 individual strings if I read right. 2 strings work fine. 3rd string doesn't. I'm assuming the lights for the 3rd string are all teed into that one cable wire. At each light, the power cable is cut and spliced with the fixture wire for each fixture and to continue on to the next. If a connection is bad at a splice, the power can't continue on. Only would need to verify voltage at first in string, or, first not functioning. Pic of wire splice:
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