[RPM] Gathering for long rituals

[FOXDIE_]

Thaumatology: Ritual Path Magic says this on pg. 20:

At every third attempt for a given ritual, apply a cumulative -1 to skill – long rituals are more difficult.

Does this mean after every third attempt, or that the third attempt gets the minus -1?

In other words, which set of numbers does a person with an effective skill of 18 gathering for a long ritual roll against:

1) 18, 18, 18, 17, 17, 17, 16, 16, 16, 15, etc
or
2) 18, 18, 17, 17, 17, 16, 16, 16, 15, etc
or possibly even
3) 18, 18, 17, 17, 16, 16, 15, 15, etc

Just a little confused on the wording.