How do i place a string after a number of spaces using sed?

[hawksbill]

Man page for sed: https://www.lightnetics.com/post/3334

The sample copy of the file:

This is my first IP 10.50.20.1
grapes
apples
pears
This is the first block
oranges
PLUMS

This is my second IP 10.50.20.2
This is my duplicate IP 10.50.20.2
grapes
apples
pears
This is the second block
oranges
PLUMS

This is my third IP 10.50.20.3
grapes
apples
pears
This is the third block
oranges
plums

Here's we adding a string after the third space. The word "great" is placed after the third space.

$ cat samplefile | sed 's/ /&great /3'