We've got 3.
I was a little bit "oh, I guess that's pretty permanent" and felt a tingle of sadness at the time when DH had his. The youngest were 1 at the time.
Now knowing what it is to have 3DC and a proper career, and to have the resources to give each child the right time, attention, quality life, opportunity, education etc, there's no way 4 would be fair in our circumstances. That tinge of sadness is replaced with the reality that we are raising 3 children well, instead of raising 4, 5, 6 in a more disadvantaged way.
There'll be someone along soon to say that their 9 children who live in a shoe are the happiest children on the planet and how dare anyone suggest otherwise. But back in the real world, each child needs their own space, not crammed in a room with 2 others as their only respite from the main areas of the home, they need food, clothes, a good school, opportunities of further education, different school trips, different social activities, and critically your one to one time.
There are not enough hours in the day for all the things our children want or often just need to do. That's before you look at what DH or I might like to add in.
OP, your children are so small this hasn't really cropped up for you yet. It's not so much you'll be glad you've never had a 4th+ but you'll one day you'll be running one to ballet, with one at a tutor, another at a birthday party and suddenly realise that another little bod in the mix would be almost impossible.