We wanted an 18 month gap, but got 24 months because that is how long it took to get pregnant (I breast fed, periods didn't come back until DC1 was 12 months, then still took a few months to get pregnant).
I have a DC3 too, with a gap of just over 3.5 years from DC2 (just over 5.5 from DC1).
A small gap is way, way, way easier IMO! Especially if you get a non sleeper!
If you have a school aged child you are tied to their routine esp. school run but also extra curricular, friends around etc, and you have two children with absolutely and completely different needs, interests and routines for years and years.
If you have DC2 when DC1 is still tiny they know no different really, adapt faster, you can have a lazy day of playdoh/ colouring/ Duplo/ cbeebies in your PJs or a summer garden day with a little sandpit and a bucket of water to wash pebbles in all day when you need to due to morning sickness or in late pregnancy if the baby is a poor sleeper, toddlers groups are easy to bundle a baby along to, and they play together and have the same interests in terms of toddler activities, days out, holidays etc. far sooner.
I found it millions of times easier accommodating toddler and baby than young infant school child, preschooler and baby. Especially hated those days when the baby had a bad night but finally settled properly into a deep sleep near morning but then had to be dragged out on the school run first thing!
Also my DSs (who have a 3.5 year gap) didn't genuinely play together properly as peers until they were about 4 and 7 - before that the younger one is being a "good" older sibling and playing with the younger one, but they are not remotely on anything like the same level. DC1 and 2 with a smaller gap were thick as thieves and genuinely best friends very much earlier. With a big gap you have years and years during which you have to keep the little one occupied away from the big one to stop them ruining their game/ lego/ painting, instead of them both doing the same things and being on the same level quickly.