b.1. Only the signal from the two sensors were used. No. Single subject (signal)/Plural verb (were).
2. Only the signal from the two sensors was used. OK. Single subject/Singular verb.
3. Only the signals from the two sensors were used. OK. Plural subject/Plural verb.
You can assume the sensors are two thermometers. The signal is the voltage values to represent the temperature. There were two sensors at two places.
Which one is right?
Student or Learner