Is three rounds enough to determine the true winner of a non-title fight?
Yes. The drawbacks of having fights last longer far outweigh the positives here, which is to say that a pivotal grant of the promotional appeal of, say, your typical UFC card is that they promote 5-6 fights on their broadcast - providing more breathing room after the main event. Extending the duration of the fight devolves MMA's current system into the Boxing imbroglio where there are few guaranteed fights to be aired and only 2-3 promoted overall. Which even at that also has negative connotations solely because not all fights are evenly matched or competitive on paper. If Zuffa is given a 3-hour window of broadcasting time, and the duration of each fight was 25 minutes, between actual fights, entrances, pre-fight videos, etc., fight cards would need to be cut down as a result - meaning less talent to your disposal - and *possibly* less fights showcased. It makes even less sense nowadays because they are a plethora of young burgeoning fighters making their way up the ranks that don't need to compete in five round fights.
Would it make things more exciting if every fight went five rounds implemented under Unified rules? More often than not it won't namely because it's widespread to see lesser experienced fighters fall to exhaustion, fatigue, etc., as the fight progresses - and quite possibly less engaging knowing that a 25 minute fight entails a more tempered pace. Even in three round fights you often see fighters fade midway through and if it's any consolation, 15 minutes of fighting in a ring/cage should be more than enough time to objectively decide a winner. This isn't long division: in a 15 minute fight more often than not, in any capacity, you have the ability to favor one fighter over another given the judging criteria.
The UFC's profit margin also goes down exponentially considering the casual fan isn't likely to fork away 50 bucks only to watch three fights give or take. Additionally, it would also cut their footprint in half considering the total number of fights on a card would go down by virtue of fight duration. Which also presents the question, if all non-title fights became 25 minutes in length, what would be the novelty in having 25 minute title fights? Would they become seven rounders?
Another detriment is fighter longevity, in that fighting an extended period of time and taking more and more punishment to the dome and elsewhere doesn't benefit anyone in the long run - and also increases the likelihood of injury. More punishment equates to more injuries and is taxing to your future as a professional fighter, which is precisely why five round fights should be saved for title bouts by default.
By the same token, an interesting exception would be five round fights specifically for main events. The platitudes of having every fight contested in 25 minute tilts under Unified rules would be averted considering the main event is far removed from every other fight on the card and treated with novelty. The current system is perfectly kosher without the imagination of extending time, insofar as having an exception for the main event that places a little more stock in the value of a fight card.