Quick take
Use G3-N1CF for the first stage, G3-N3CB for carbon, and WD-G3-N2RO for the 400 GPD RO membrane. The RO membrane is currently $92.99 with a 24-month / 2,200-gallon rating.
Correct cartridges
Waterdrop identifies G3-N1CF and G3-N3CB for the G3 family, plus WD-G3-N2RO for the 400 GPD G3P400 system.
Replacement intervals
CF is typically up to six months, CB up to 12 months and WD-G3-N2RO up to 24 months under manufacturer guidance.
Current membrane price
WD-G3-N2RO is currently listed at $92.99 one-time, with subscription pricing sometimes lower.
Annualized core-filter cost
Using current list prices and nominal schedules, the three core cartridges annualize to about $136 per year.