The effectiveness of personnel planning significantly relies on organizations’ capacity to address issues of overstaffing and understaffing resulting from uncertain and seasonal demand. This underscores the importance of developing and evaluating new and enhanced labor flexibility strategies aimed at shaping a workforce capable of meeting unpredictable demand patterns. This study investigates a triple flexibility strategy that integrates multiskilled employees, annualized working hour schemes, and overtime, all while considering demand uncertainty. Conducted at a retail store in Chile, the research employs a two-stage stochastic optimization approach to determine the optimal staff size and training plan. Additionally, the study compares the optimal solution provided by the TSSO approach with alternative myopic approaches. The findings highlight that the TSSO approach offers the most cost-effective solution, achieving superior demand coverage while minimizing salary, over/understaffing, and training expenses.