merge 2 files with powershell?

I have 2 files

A.txt

this is a test line 1 A file

This is 2nd line A file

This is end lines A file

B.txt

This is 1st line B file

This is 3rd lines B file

This is 4rd lines

.....

This is 1000 lines+ in B file

Now how do I merge A file in B file at line 3 (remove line 3 from file B and add all lines file A to B file at line 3 (replace that line 3)

Combine.txt

This is 1st line B file

this is a test line 1 A file

This is 2nd line A file

This is end lines A file

This is 4rd lines

.....

This is 1000 lines+

====

How do you do that?

Thanks,