I have three dcs.
Dd1 was 3years and 10 months, ds 22 months when dd2 was born, so I had a few months of 3 under 4.
It was hard hard work for a long time, but I was physically very unwell and also had PND.
Now they are 8, 6 and 4. Dd2 has just started school. It is great now. they bicker, fight, rub each other up the wrong way sometimes but are basically great. I am so glad I had the three close together.
But when you hear them playing nicely now you forget all the hard bits. Be nice to yourself, like others have said, wine, chocolate, Radio 4 and an occasional G&T are still very useful.
Get used to doing things together with them - I was very good at bathing all 3 at once, and in fact dh used to take them all in the bath with him at once! that was hilarious.
If you get help from anyone, like family or friends, then don't worry abouthousework etc - its going to be a total guddle for a while - and spend some quality time with them one-on-one - we found that to be essential.
Hope that helps a bit. Good luck