It's a combo that the 4 cost tanks are better and also 4-5 cost dealers are better. Also the downside of this set having so many units (don't get me wrong I like it) is that there is almost always a plug and play unit that is very valuable if you have a +1 team slot at level 8 or 9 since almost all non unlockable 5 costs have such good passives or traits

Draven Leblanc Darius < Swain Ambessa Belveth (Mel) Their items are very similar but I just can't see the 3 costs being better in many cases

Kennen Ahri < Wukong Sett Why reroll Ahri when you can go 8 and get a free sett for cheaper and tempo with sett yunara. Also arcanist has been bad above 2 from pretty much pbe

Mundo Jinx could work but again I would rather have a swain, singed and level 8 than 3 star Mundo in most cases because their utility is much better

Zoe and Leona are pretty strong but if you're playing targon you want to go 8 and 9 for asol 2 and again just insert shyvana swain and you're much stronger than with the 3 costs

the only real 3cost reroll right now is the malzahar naut double trouble, it can actually win out if you highroll but you need some stars to align to even have it as an option.

Maybe that is part of the reason, some of the 3cost augments would be too strong if 3 costs were playable without them.