The order-disorder transition has been studied in two AB(3) miktoarm stars of styrene and isoprene with SAXS and theology. A mean-field theory has been developed to treat the most general case of miktoarm stars : A(m)B(n) (m not equal n) which provides the phase stability criteria and the static structure factor in the disordered phase. The theory predicts a maximum in the critical value of chi N-t (N-t=N-A+ nN(B)) as a function of the number of arms n in AB(n), for n=3. This maximum arises from a delicate balance between the stretching free energies of the A and B blocks forming the miktoarm stars. The theoretical predictions for the phase stability and the structure factor have been tested experimentally, using SAXS, and the interaction parameter has been extracted. The latter exhibits a weak T-dependence which is consistent with fluctuation effects near the transition. Further evidences for the existence of fluctuations at temperatures above and below the transition are provided by (i) the nonlinearity of the inverse peak intensity vs inverse temperature, in the disordered state, and (ii) the existence of metastable states in the ordered state. All SAXS peak parameters were discontinuous at the order-to-disorder transition, revealing a first-order transition.