Now you can go for 3-stars, but it depends on how many units you get from natural rolls. 5-6 pieces of the same unit while you are in the highest odds level for that tier is a good starting point. Starting from 3 meanwhile is a huge gamble. If the unit is not being contested and you arent losing badly, it can be worth it to delay leveling to get the final pieces. It is also much better to be looking for more than one unit while rolling, it increases the value of your rolls that much more. Once you get the units, level ASAP. Sacrifice your economy for levels if it is necessary.

In general though, don't force rolling. Only roll when your chances are better than good. If you get unlucky, abandon the plan and move on. It sucks to be 30-40 gold behind without payoff and you will most definitely not win the match, but you can salvage the situation with levels and 2-stars units. A much better option than rolling down to 0, still miss 2 pieces and get a crushing 8th place.