Each person carries some chance of carrying an STI. Some people aren't aware of their status or may deliberately hide it. For those two probabilistic variables alone, while the absolute risk may be small, it's still at least double that of 2-people relationships. If someone is more likely to enter a 3-person relationship after leaving another 3-person relationship, you have added network effects increasing the risk. How could it be true that the risk isn't different? What have I missed?
You're assuming ceteris paribus, but that isn't a given.
People in poly relationships tend to know their status better than monogamous cohorts. Thus, even if one partner has an incurable STI, they're usually aware that they have it, and keep their viral load undetectable (which prevents transmission), so the factors round the risk down to the same very low range, despite more people involved.
Polyamory requires communication and consent, a healthy polycule is no different than a healthy monogamous relationship when it comes to the expectation that your partner(s) will care for their sexual health and make decisions with you in mind. In one way you are correct, more people involved means an increased possibility that someone will be dishonest or take risks that will expose you. However, in the same way that bringing an STD into a monogamous relationship has consequences so would bringing an STD into a polyamorous relationship. You risk damaging your standing in the relationship.
Maybe on a per member basis, or theoretically. The risk in a monogamous relationship is if a member acts outside the relationship. The more people you add to a relationship, the more that risk increases.