Jackfruit, pumpkin, and green banana are alternative flour sources with potential for use in the bakery industry, helping overcome the drawbacks of traditional wheat flour and improving the nutritional value of snack foods. The use of composite flour appears to be a promising approach for bakery products, substituting for wheat flour while preserving consumer acceptability. This study formulated a composite sandwich snack using jackfruit (Artocarpus heterophyllus), pumpkin (Cucurbita maxima), and green banana (Musa acuminata) flours blended with wheat flour. Flours were prepared by hot-air- drying at 60°C. Proximate analysis revealed pumpkin flour contained 8.75% crude fiber, while jackfruit flour had 4.86% protein. Banana flour exhibited the highest Total Phenolic Content at 0.81 mg GAE/g, whereas pumpkin flour showed the greatest Total Flavonoid Content at 1.77 mg QE/g. Sensory evaluation by a trained panel determined that the optimal Formulation comprised 30% jackfruit, 15% pumpkin, and 25% green banana flour layers, combined with an oyster (Pleurotus ostreatus) mushroom filling (selected from separate, single-flour sensory optimizations rather than a simultaneous three-component comparison; see Section 3.5). The final developed snack yielded an impressive 10.31% crude protein, 6.18% crude fat, 8.16% dietary fiber, 42.82% carbohydrate, and 264.14 kcal/100g on a dry-weight basis. The snack also displayed significant antioxidant capacity, with TPC of 0.31 mg GAE/g, TFC of 0.112 mg QE/g, FRAP of 0.06 mg TE/g, and DPPH half-maximal inhibitory concentration (IC50) of 318 ± 102 µg/mL. In vitro starch digestibility testing resulted in a predicted Glycemic Index(pGI) of 68.11, categorizing the formulated composite sandwich as an intermediate-pGI food (56–69 range). The snack is not gluten-free, as it retains a majority wheat-flour base; it offers a nutrient-dense, fibre-rich alternative for the bakery sector, although the predicted GI is based on an in vitro assay and requires clinical validation before health claims are extended.