I run a few 3-color cards in my cube:
All of them are impactful when they hit the battlefield. The problem with cheap 3-color cards is that your mana base has to be able to support aggressive cards in three colors
and their effect on the battlefield often just isn't impactful enough to justify that third color. With expensive 3-color cards it's easier to just splash your third color plus you can expect more powerful things at a higher mana cost making the card actually worth splashing for. So, what I'm looking for in this set is 5+ mana 3-color cards that make me want to splash that third color.
Narset, Enlightened Master has a powerful effect, but the body is underwhelming, and I'm not too fond of hexproof (although it might be okay on a six mana 3/2).
Zurgo Helmsmasher looks fun. Definitely a playable curve topper that I'ld want to splash a color for in a semi-aggressive deck.
Sage of the Inward Eye might be a better cube fit than Narset in its colors actually. Lifelink for your whole team is powerful, and the 3/4 evasive body is nice. Not better than Archangel of Thune though, and that's in one color.
Duneblast is awesome, but at seven mana just one mana too expensive.
Ainok Bond-Kin should have been a human. Cool common nonetheless.
End Hostilities is interesting.
Suspension Field might replace Journey to Nowhere over here.
Wingmate Roc is replacing Archangel of Thune because it has more interesting applications.
Thousand Winds looks interting. Sometimes we forget how good a
Mahamoti Djinn is in limited formats, and this one has a definite upside.
Clever Impersonator is going in.
Jeskai Elder looks more interesting than the shadow looter.
Ruthless Ripper is awesome, but I would want at least two morphs per color in my cube and watch the 3 cmc slot carefully.
Mardu Ambusher is an aggressive Ravenous Rats in a good tribe.
Horde Ambusher is playable.
Sarkhan, the Dragonspeaker looks like a fine replacement for one of the two red planeswalkers I currently run. Koth can be a bit overwhelming sometimes, Sarkhan looks a little less cutthroat. On the other hand, mono red still has to win a draft...
War-Name Aspirant is good. Human and +1/+1 counters.
Hardened Scales is probably going in.
Icefeather Aven is a good card.