Everyday decision-making often involves choosing between multiple alternatives, and developing unified theories for this process would benefit broad disciplines of behavioral science. A key challenge lies in explaining behavioral irrationalities that arise specifically in multi-alternative decisions. This study, based on pre-registered procedures, investigated such non-normative behaviors in a three-choice dot numerosity discrimination task. We systematically manipulated the value of the lowest-numerosity alternative (the dud stimulus) and quantified its effects on choice, response time, and confidence. We then explained observed behaviors with a sequential evidence accumulation model featuring a “max vs. next” decision algorithm. This pseudo-optimal decisionmaking rule naturally accounted for a well-known non-normative choice pattern: the violation of the independence of irrelevant alternatives. Our model also explained a multi-choice metacognitive bias, known as the dud-alternative confidence boost, by identifying two critical time windows in confidence constructions: confidence is based on sensory evidence collected shortly after stimulus presentation and evidence gathered right after decision-making. These findings represent a significant advancement in understanding the dynamics underlying multi-choice irrationalities.