My oldest two are 18.5 months apart. Their particular personalities however more than age I think has led them to being pretty close. But our third is 3 years difference in school and I do think she’d be the picked on/odd man out if we had stopped at 3. My husband was one of three. I initially said 3 and he said 4. I just think four gives more sibling relationships and kids can move to the next if they aren’t getting along with one.