The odds are not in their favor (unless...)
There are a number of very good "frame challenge" answers to this question: answers that suggest why this method might not be fun for the players, or fit the expectations of the game. With the understanding that those answers are an important part of this conversation, let's also take a look at the mathematical likelihood that your players succeed at the task that you've given them.
Let p= probability of a single success on a single roll. (So 1-p = probability of failure on a single roll).
To calculate the probability of 5 successes before 3 failures, we can calculate the following:
1. Seven rolls, with 2 failures and 5 successes (where the last roll is a success)
The probability of this will be:
(Here, 6C2 means "6 choose 2", which accounts for the different places the two failures could go in the first six rolls. It is equal to 6 * 5/(2 * 1) = 15)
2. Six rolls with 1 failure and 5 successes (where the last roll is a success)
The probability of this is:
=5 * p5(1-p)1
3. Five rolls, all of which are successes.
Probability of this is: p5
If we sum the above probabilities, we get the probability of 5 successes before 3 failures:
p5(1-p)2(15)+5 p5(1-p)1 + p5
Ok... is that good?
Depends on how likely they are to succeed on a single roll.
In the comments attached to your question, you said that the two characters who could attempt this have "+1 to Arcana," and that this would be a DC 15 check. Assuming that means their total roll will be 1d20+1 (e.g. the sorcerer has an Intelligence of 12, the bard has an intelligence of 10, and neither is proficient in Arcana [allowing the bard to get +1 from their Jack of All Trades class feature]), then p= 7/20. In this case, their total probability of getting 5 successes before 3 failures is 5.56% (about the same as if this were a single DC 21 check).
Now, perhaps these characters could attempt to improve their odds, and one of the two (either the Bard or the Sorcerer) could use the Help action each turn, giving the other advantage on the checks. In that case, p= 1-(13/20)2 (that is, the probability that you don't fail twice on two rolls). In that case the total probability of 5 successes before 3 failures goes up to 37.192%. Still worse than a coin flip, but a lot more likely than before.
NOTE: If these characters were proficient in Arcana (bringing the total check up to 1d20+4), their odds of getting 5 successes before 3 failures increase considerably (to 22.7% normally, 75.64% if they are using the Help action each turn). But given your description of them having "+1 to Arcana," I'm assuming this is not the case.
The exact influence of Bardic inspiration is hard to calculate here, because they will likely run out before the task is finished (and we're not sure how many successes they will still need when they run out). But if they had Bardic Inspiration on every single roll (they won't), that would add an average of 4.5 to each attempt, bringing p up to .575. This will change the probability of 5 successes before 3 failures to 36.67% (or 85.62% using the Help action).
And if you have a Cleric who can cast Guidance every turn (giving an extra 1d4 to one check per turn), that will influence things as well. But if all these things are happening simultaneously (Help Action, Bardic Inspiration, Guidance, and someone making the actual check), then three of the six PCs are using their actions every turn to influence (or make) this check. That means the party is at half strength to deal with whatever else is going on (for example, an angry mob of villagers attempting to bust the door down and take your NPC). That being said, all of these factors together would bring the probability of five successes before 3 failures up to about 98%.
Bottom line, the probability of success here massively changes depending on the resources the PCs bring to bear in this contest, and the strategies they use. And what they have available is very hard to predict, since we're not sure how many resources they will spend on their way to this circle, or if they will even attempt to go there. It's also quite possible they will change their tactics part way through this attempt. If you're trying to balance this check, it will be hard to do ahead of time.