Sentences in bold are used to indicate what's new in each stage.
In Stage 3, Flower Garden has the following effects:
General Effects
Grass moves increase in base power to 1.3x (+30%)
Fire moves increase in base power to 1.5x (+50%)
Abilities Affected
Flower Gift and Leaf Guard are activated
Harvest will always find a berry at the end of each turn
Swarm is activated, and will now raise the base power of Bug moves by 1.8x (+80%)
Overgrow is activated at all times
Flower Veil will also reduce damage done x0.75 (-25%) to the bearer and allied Grass types , in addition to prevent allied Grass-types from any of their stats being lowered
Moves Affected
Transitions to other Field Effects
Moves that will grow this field to Stage 4
Abilities that will grow the field to Stage 4 on switch in
Other moves
Moves that will ignite this field in the absence of Rain or Water Sport and turn it into a Burning Field
Seed
The Synthetic Seed boosts Special Defense, applies Ingrain to the user and grows the field by one stage.