For Tier 3 cards to use your free unlock on, it really depends on what decks you want but I'll tell you which ones I prioritized that are easy plug and plays for getting into Pool 3 early.
- Doctor Doom - He's essentially a 6 cost 15 power because he summons two other Doom Bots that also have 5 power. He was my instant unlock because you can plug and play him in almost any deck, perfect if you're just getting into Pool 3.
- Patriot - He buffs all your cards with no abilities by 2 power and it's On-going, so you can combo that with Onslaught to give 4 power instead. You already have almost all the cards necessary to play him and the patriot deck is fairly consistent.
- She-Hulk - She's a 6 cost with 9 power, however because of her ability to can technically "cheat" her out early on turn 4 if you don't play anything by turn 4 because her cost goes down 1 less for each unspent Energy last turn. Perfect to combo with Sunspot.
- Magneto - Think of him as a better version of Hulk. He costs and has the same power as Hulk, but he has the ability to disrupt your opponents field because he can move opponents other 3 or 4 costs from one location to where he's played.
- Magik - She's just so fun to play with because she basically turns a 6 turn game into a 7 turn game and can get rid of an unfavorable location at the same time. She was so hard to use back then because she was a 5 cost, but now that she's a 3 cost, you can use her ability more consistently now.