A stratified random sample of a population is designed to ensure representation of subgroups. If a population of $N = 1000$ consists of three strata with sizes $N_1 = 500$, $N_2 = 300$, and $N_3 = 200$, and we require a total sample size $n = 100$ using proportional allocation, how many individuals must be selected from each stratum ($n_1, n_2, n_3$)?