Why boy, are you not interested in what 3yo girls might be getting?
I have a son and daughter. When DS was 3 he got a bike (balance bike the Christmas before he turned 3, pedal bike the Christmas before he turned 4). Gave DD the balance bike for her third birthday. She's obsessed with paw patrol so also got various paw patrol gifts. Play dough was a big hit.
On my ideas list for Christmas I have:
Duplo train set
Vtech Kidizoom camera (DS has one and she often tries to steal it)
Toy microphone
I won't buy all these, but my parents usually ask me for ideas, so I will probably add to the list and decide what I'm getting nearer the time. I don't buy too early as I wait and see what they're into towards end of November.