I think the most important thing for the junker (as one of the most versatile ships) is that it showcase a variety of playstyles while (and this is important) remaining self-sufficient. These are Novice matches and, while I'd love to put emphasis on good team play, I think that expecting Novice matches to have the coordination required to make support ships function is a bit optimistic.
Build 1:
Gatling front and both sides on top deck
Mortar both sides below
I'm a little hesitant about triple gatling (I feel an Artemis or Banshee might make for a more well-rounded ship), but my biggest complaint is that I'd love to see the gatling and mortars switched. It might be personal preference, but most Junkers I've been on run the gunner belowdecks, and keeping up consistent gatling fire at the hull is more important than constantly firing mortars (as you really need to wait for the hull to drop...but we all know that). This would allow the Engineers to do their thing while the gunner fires the gats and keeps the main engine running. I think that change would be to the build's benefit, but it's not a huge deal.
Build 2:
Artemis front and top left
Flak on bottom left.
2 Gatling guns on the right
This one bothers me. Double gatling on a side while requiring a trifecta for explosive finishing damage is perhaps a bit demanding for newer pilots, and the double explosive damage left side is...well, I think it's a bad idea.
I'd much rather see a triple Artemis (front and left side), as this is a bit more in vogue and also a bit more effective than Artemis/Flak (at least when the crew might not know that the Flak is best against a downed hull). The double gat isn't as bad then...but Gat/Mortar might still be a better use of that side, allowing novice players to experiment with a hybrid ranged/brawler junker (which is quite popular).
Build 3:
Artemis front.
Gatling both sides on top deck
bottom Deck is left Mortar, right Flak
This one has two issues. Firstly, it's far to similar to the first build. Secondly, the new range changes to the gatling have made gatling/flak just bad. I don't expect newer players (especially in games heavy with Pyramidions) to understand that they have to fight with the gat/flak side within 400m but outside of 220m for both guns to be effective. That's asking for rather a lot of game knowledge and piloting skill.
Personally, I wouldn't mind seeing a Banshee build, or a Carronade/Flamer ship...something with a bit of disabling power that showcases some of these easy-to-use and fun weapons with a bit more of a niche than "killing power." Banshees, Carronades, and Flamethrowers are all very simple weapons that are easy to use and to understand, and I think a third ship build using some or all of these would really round out the available options.