actually the chances for that to happen are low but not impossible. you have 3 assassin 5* VHE rainmaker and ACOTD chance for 3 rainnaker are 0,33^= 3,7% (1,2% for 4 rainmakers in a row)
this is very unlucky and i like your idea of setting dupe caps. or let us just farm hard for specific chars like in other games.
in marvel strike force you can play a specific level 5 times per day which drops fragments of specific chars...so you will get em all, but it will take a ton of time...love the system, but the game economy and the environment are different there....