Your answers are not helpful. Different castes and sub-castes have different applicable rules.
All I can say with confidence is that the shraaddha thithi is Mithuna-maasa Krishna-paksha prathama. The first three maasikams are on 8.8.17, 7.9.17 and 6.10.17. The dates 21 to 25 September 2017 have no relevance, even for oonam, shoda-kumbham or maasikam.
Can you say what the "3rd month prayer" consists of? To which deity or deities? To the ancestors? Any water-offerings? Any fire-sacrifice? Any feeding of crows or cows? Any feeding of Brahmins? If so, how many?
Since you use the name Joslyn are you or either of your parents Christian? Or follow tribal practices?
S Narayanaswamy Iyer