Autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), and developmental language disorder (DLD) are neurodevelopmental conditions (NDCs) that share etiological factors and frequently co-occur. Despite this, they have rarely been studied together?particularly in relation to functional outcomes. In this study, we investigate the association between the developmental pattern of sustained visual attention in infancy and number of diagnoses, and map the clinical profile of 6-year-old children. A cohort of 6-year-olds, originally recruited in infancy due to elevated (n = 42) or low (n =7) likelihood of ASD, were assessed for sustained visual attention, diagnostic outcomes, general adaptive functioning, intellectual abilities, and language skills. Participants were grouped based on the number of NDC diagnoses (ASD, ADHD, DLD, and/or Subthreshold ASD) they received at follow-up. We could not find statistical support for an association between sustained visual attention and number of diagnoses. Findings revealed no significant differences in adaptive functioning, intellectual abilities, or language skills between children with no diagnosis (n = 24) and those with a single diagnosis (n = 15). However, children with two or more diagnoses (n = 10) scored significantly lower in general adaptive functioning, intellectual ability, language production, and verbal comprehension compared to those with only one or no diagnosis. The results indicate that compared to children with only one diagnosis or no diagnosis, children with two or more diagnoses scored lower on several key functional domains, emphasizing the need to prioritize children with multiple diagnoses or confirmed functional impairment in clinical settings. Moreover, the findings indicate that a single diagnosis in preschool-aged children should not be a stand-alone outcome measure in sibling studies, if the goal is to identify early processes that predict meaningful differences in everyday functioning.