collecting 3 petals each time from any single shrub is perfectly correct. you fused 3 flowers into one. it's not luck.. it's right. extra drops more than 3 is luck. any single flower does have lucky drops of 2. add them together.. and sometimes! u should receive them.
it's only the Red Trillium Shrub that I don't see with extra lucky drops.. max I've seen is 4.