You may find that by the time you are putting him down to sleep, he is already overtired and once that happens babies tend to get even harder to get to sleep.
It's a bit of an art form to catch them when they're tired, but not overtired. It comes with experience and practice.
Having said that, some babies can do it, and some can't. 3 months is still pretty young, but DS2 was doing it by then, will DS2 started around 4 months.
What are the signs of tiredness? so far he started to scab his eyes when he is sleepy but that doesn't happen a lot, he also cries with closed eyes and that's when I know it's time for bed. the obvious signs are when he gets agitated and that's when I'm too late. other than that I have no idea.