That's the part that kind of worries me about the third floor. Once you get to convinces with 6 or more options, the odds of success drastically reduce compared to 5 or less. I know I can do the third floor for multiple weeks but I don't think that is sustainable every week unless I am willing to pay the 25 diamonds more often than I feel I should because I am going to have multiple convince failures. Of course, I will fight the encounters that have favorable matchups every chance I can so that helps. One thing I was thinking was making my decision based off the rewards I get from the first two floors. If I am winning a lot of diamonds that week, I might make a move on the 3rd floor. Otherwise, I might hold off that week and build up my resources. I would happily spend the diamonds won from the gold spire rewards for extra turns as this would help save me plenty of resources and hit floor 3 each week so that I could win more of the better prizes like genies as you mentioned.
Note: I am still just a chapter 4 player so I'm hoping down the road I will have a stronger military and resource income to consistently complete floor 3 each week.
I've gotten to the top every week for months now, and I usually cater the top (sometimes fight, but that's only when I'm doing a push week on tourney and have all my five-days out and fire fed.) The most important thing to remember is that if you don't have a good hand after the first try, withdraw. Don't take that second and third try ... you'll just throw good stuff after bad. Withdraw and then try again with a new round of three tries. By "good," I mean items unneeded ... not green check marks.
If you've got, say, 7 items and 5 spirits, the last thing you want is to get 4 green check marks and one no one needs it. That means that you have two tries left and have six items to choose from for the last spirit ... a dismal one in three chance of winning. However, if you get on the first try three "no one needs it" and 2 "wrong item," well, that's a lot better. Now you have only four items left and five spirits, and you know that two have already rejected two items, so for those two spirits, you have a two tries and three items left to choose from ... double the chance of the first scenario. And for the other three spirits, you know what two of them want already (though you don't know which two, but still, much better odds.
Anyway, remember to withdraw ... it's easier to lose goods than it is to lose diamonds, or at least it hurts less. There have been times when I've spent up to 75 diamonds to get to the top of the spire (actually up to 100, as long as I won at least 25 along the way, so 75 overall). That doesn't include the diamonds I'll get back when my team gets to either silver or gold. If I spent up to 75 and we only get silver, well, I've gained 50 diamonds for the week plus all the other goodies. But most weeks I actually don't spend any diamonds on my way up. I just keep withdrawing until I get something I like .... on a big boss I've been known to withdraw 6 times, if the odds are too dismal otherwise. Yes, it's expensive, but look at it this way ... withdrawing six times on the first try and then going on to win means that it takes 8 total guesses to win. That's not that bad, when you're talking about the big boss at the time. I would never have to do that much on a lower boss, so it works out. Good luck.