What makes you think that?
I can't remember to be honest when I knew. But I'd be very cautious 3 months in especially if you've had bad relationships previously. Do you know about love bombing and future faking?
One of the biggest mistakes you can make in a relationship is to latch onto one thing that's been an issue in previous relationship(s) and decide that because your current bloke is the opposite of that specific issue, that means they are great and you'll stay with them for a long time.
Another red flag that you're falling too far too soon is a sense of scarcity, feeling that you've landed a rare prize and you'd better hold onto him quick before he leaves or anyone else notices because not many men are like this.
Or if you've been with several level 8 bastards, a level 3 one will feel like a prince, but the only acceptable level of bastardness is none.
I am not judging these things BTW, I have done all of them.
Green flags on the other hand - I remember one of the earliest things I noticed about DH was that I could completely relax and be myself around him, I didn't have to adopt any role or put on any performance.
When we argued/disagreed, (over the freshness of eggs!) at the time I said it was weird because it felt like arguing with my sister. Later with context I realise that what I meant was that disagreeing with him was not scary because it didn't feel like it could lead to a break up. Like my sister would never stop being my sister just because we were annoyed with each other, the entire strength and value of our relationship was never called into question over a minor disagreement.
What is his worst trait? Green flag here is that the worst trait is something that is mildly annoying. Red flag would be things like anger problems, drug/alcohol problems, critical streak etc.