The first clue lies in the word 'Flower' itself,
Flower starts with letter 'F' which stands for Fabaceae.
Now the trick for remember the 3 families of Flowering plant:
Lily Flower in Soil
- Lily - Liliaceae
- Flower - Fabaceae
- Soil - Solanaceae
Note that all the words have common ending - 'aceae' (may be like a funny surname to indicate they are different sub-families from same old 'flower' family)