While writing the original comment, I quickly browsed the LIGO publications list[0], but nothing jumped out at me as being about the candidate, specifically. The papers with "candidate" in the titles are pre-upgrade, so I don't think they include the candidate that was referenced in the seminar I attended.
EDIT: Found something. The third candidate event (LVT151012) is mentioned in this paper, in the 2nd paragraph of the introduction and discussed in more detail in Section C (beginning on page 11): https://arxiv.org/abs/1606.04856 ("Binary Black Hole Mergers in the first Advanced LIGO Observing Run", LIGO+VIRGO).
[0] https://www.lsc-group.phys.uwm.edu/ppcomm/Papers.html
EDIT: Found something. The third candidate event (LVT151012) is mentioned in this paper, in the 2nd paragraph of the introduction and discussed in more detail in Section C (beginning on page 11): https://arxiv.org/abs/1606.04856 ("Binary Black Hole Mergers in the first Advanced LIGO Observing Run", LIGO+VIRGO).