Because the verb must agree with the subject. Nő is in singular, so kijön is also in singular.

But if you don't use the number, you have to put the whole sentence in plural: "A nők kijönnek a kertbe." = "The women come out into the garden."

...The three women come out into the garden...

--------- once you mention a number or quantity greater than one, everything beyond it switches to the singular . . .

Big 14 mar 19