3 weeks and 6 days is early to know your pregnant, congratulations and I bet your sooo excited!! Having a baby is the most rewarding experience in the whole wide world.
I started showing at 5 months with my first baby. It does differ between women and I’m sure women who eat more or have already had a child would show faster. You do get a little belly at 4 months but not enough to wear maternity clothes. That was the transition month for me haha My regular clothes weren’t fitting well but maternity clothes were too big! Good luck with your growing baby/belly hehe!!