How much rent is owed right now?
If rent is weekly or fortnightly, 3 months is roughly 13 weeks.
The arrears grounds explained
Ground 8 is the strongest: it is mandatory, so if it's proved the court must give you possession. It needs at least 3 months' rent owed (13 weeks if weekly or fortnightly) both when you serve the notice and at the hearing.
Grounds 10 (some rent owed) and 11 (persistently late paying) are discretionary: the judge decides whether possession is reasonable. Landlords usually use all three together, so the claim can still succeed if the tenant pays just enough to drop below the Ground 8 threshold.
All three arrears grounds need 4 weeks' notice.