Ignoring Shiki, the only fights which would really count in this gaunlet may be these ones(i think they would rape the rest without effort)
Hancock
Yami Teach
Kuma
Doflamingo
Mihawk
For Hancock it's because i'm not certain of her placement, but at best, i have her slightly weaker than Doflamingo and at worse she'd be around Sanji level, in other words, a huge range in power. If she is Sanji level, they'd rape. If she's slightly weaker than Doflamingo, it could require low-mid diff for the trio to win. I have yami Teach around Doflamingo level, but more careless and Zoro not possessing a devil fruit could be a bad match-up, i'd low-mid difficulty since they didn't waste much energy on Hancock. After 2low-mid diff fight, facing Doflamingo may make them push themselves to mid difficulty especially since Law by this point would probably be at a moderate level of stamina. With Law low on stamina by the time of fighting Mihawk, he'd be taken out really quick, with Zoro following soon after and Luffy(with g4) giving Mihawk around mid difficulty
Without heal, they still reach Mihawk but give him a harder fight. Law could barely give mid diff to Doflamingo, so he is a low diff fight for Mihawk(same goes for Zoro). Luffy with g4 would still give him mid difficulty but in a team with Law and Zoro(especially with Law's swapping ability and hax), it would make things much more tricky with Mihawk probably deciding to either take Law out first or takes out either Zoro or Luffy either when Luffy is out of g4 or running out of it after stalling them for a while(if Luffy decided to start with g4), probably pushing the overall difficulty to high if not extreme