Batteries in series discharging unevenly
Assuming alkaline / zinc chloride / similar chemistry:   

If you got eg 0.5, 0.6, 1.1 then it could be explicable by the fact that there is very little energy in the area under about 1V, so that if one cell had say 5% more energy content than the other two it would still be on the "final approach" to fully flat while the other two were essentially completely exhausted.

However, a cell at 1.5V has the majority of energy capacity remaining - probably 90%+ .
For one cell to be at 1.5V while the others are fully exhausted then they would have had only 5% - 10% of their new energy content at the start of discharge.
SO this is not a batch variation - two of the batteries were very close to dead at the start of discharge OR something else has happened not mentioned in your question.

If the batteries are 'alkaline' they would retain a substantial portion of full charge for many years. If they were Zinc Chloride or other similar chemistries then they may well have very little capacity after say 2 years of shelf life.