This can be the best thing to do in order to recover some percent of the lost when you bought it @$20. If you only hold and wait till it goes back to $20, it will take long time but as techalt said that once it goes up, lets say it will rise to 5-10 or even back to 15-20 then you will gain decent profit. Otherwise you wont earn nothing if did not buy more @3.5.
Indeed, there is always a bigger risk if the price stuck at $3.5 or even going down more but it is the art of trading or we can call it as strategy on trading to recover lost.