If a guest had 3 star reviews, I would be worried. However, if I notice that they have an overall 4.5 or 4/4.5 in some categories, then I try to take a closer look at their reviews to see what the problem is.
Sometimes a guest can have several 5 star reviews, but one host has rated them low, bringing down their average. Sure, what @Jeanne224 says about hosts generally being too generous in their reviews is probably true. However, sometimes it is the host who is the problem and not the guest. I like to give people the benefit of the doubt.
So, if a guest has a negative or less than glowing review from just one host, but no other bad reviews, I usually message the guest before accepting the reservation to ask what happened. I find that people are usually quite forthcoming about any problems that occured.
Also, I will check the host's other reviews and the reviews they have left for other guests. In some circumstances, I have discovered by doing this that that particular host is super critical of other guests too, or other guests have brought up issues with the host in their reviews.
Much as we should protect ourselves (and I understand if any host prefers to accept only 5 star guests), occasionally you do get good guests who just happened to encounter a petty and difficult host. We all have our different levels of tolerance and some things that may really annoy me might not be a problem for others and vice versa. I often see posts on the forums where hosts are complaining about something guests did that really isn't a big deal for me (and I am quite strict about my house rules).