I have three under four, about 20 months between each. DC2 was a surprise, DC3 was (maybe badly) planned.
It is hard, and I wouldn't underestimate that. Practically you just get on with it because you have to. I have no family support nearby, but did have my parents staying for a couple of weeks after DC2 and 3 were born, which was invaluable in terms of help. Meant my husband could focus solely on the older kids and Mum and Dad could look after me and help with the baby.
Right now, with my eldest 3.5 and the baby coming up 6 months, I'm almost at breaking point. The brutal answer to the screaming fits at once is that one gets left (in a safe space) while you attend to the other, and back and forth it goes. In fairness though, I haven't been in that position very often.
Parenting a baby/toddler while pregnant was ok, but I never suffered much morning sickness, which helped massively. I felt guilty that I had to stop carrying my eldest during the third pregnancy, as it was just too much. They probably have more expected of them than other 3 year olds, though I try hard to be mindful of that.
The sleep deprivation is worse (eldest still didn't sleep through when DC2 was born) and in three and a half years I've had maybe half a dozen ok nights, if that. It's horrendous and has had a big impact on our relationship, including an almost total lack of sex atm. I'm hugely touched out, never without a child or baby attached and craving space to be me again (roll on a return to work!).
BUT, after all that . . . No, I don't wish I'd waited. It's been hard and stressful and some days I just fall apart in tears. But my older two are beginning to play really nicely together. They have the same interests in terms of TV, days out etc. and we can easily find things to do as a family. They both adore the baby, who is constantly stimulated and surrounded by love. In a few years they'll all be at the same school and practical life will be a little easier. We can begin getting rid of the baby stuff rather than hanging onto it for years, I'm elbow deep in nappies but have never not been so there's no sense of going backwards. It all works, kind of!
And I should say, I have the most amazing childminder who still takes the older two for a couple of days. Without that it would be much much harder and this would probably be quite a different post. So I'd say go for it if you have the support, paid for or otherwise. Maybe not the reality check you want, but that's my experience, hope it's a little helpful!