While ordinarily the previous season's away kit would become the new 3rd kit, thinking about it logically, it's unlikely to happen. In the league, away at Villa and West Ham both our home and away kits will clash, and I doubt red and black stripes would be different enough either.
Therefore, three options:
1: We go back to the white kit with the black and red sash as the new third kit, but this is unlikely as it's 3 seasons old.
2: We use last season's third kit, the dark blue one (which I think looks fucking awful) but again it'd be the third year of use so unlikely. We wore this at Villa Park last season and there was still a pretty bad clash.
3: We get a new third kit, would likely piss off us fans but a great chance for the club to flog more Umbro shirts with us moving to Nike next season. Don't forget that we'll definitely have three new kits next year due to the new manufacturer.
If the third kit does look like the black and gold mock up that's been done, the club would definitely be on to a winner. But I reckon that the monochrome badge on the home shirt is a subtle homage to St. Marks and would be surprised if they did an entire kit in St. Marks' colours.