No doubt that Jayce is the key champion. Basically, if you cannot find a Jayce in your mid-late game, you will die very soon, and if you can get a two-star Jayce, it would be a guaranteed win in most of the cases, unless you are facing a Jhin's team. Try to get a winning streak in the early game, and prepare items for Jayce first, then Orianna and Janna. If you have Tactician's Crown with you or you are lucky enough to reach level 9, Yuumi will be a good choice.
Jayce basically needs tanky items. Other than those recommended, Ionic Spark (strong) / Sunfire Cape / Dragon's Claw / Gargoyle Stoneplate will also be good.
Orianna needs attack items of course, and Spear of Shojin is kinda important for her. Rabadon's Deathcap, Giant Slayer, and Archangel's Staff are some of the good choices.
Try to give Janna 1-2 Chalice of Power, it really helps a lot.
If you got extra items, ap items go to Heimerdinger, tanky items go to Taric, and Mana items go to Seraphine.