How to delete some rows in a file by using bash file?

How to delete some rows in a file by using bash file?

I want to delete some specific rows in a file. For example I have a file
containing these rows :
+1 1:-51.000000
+1 1:-47.000000
+1 1:-53.000000
+1 1:-48.000000
+1 1:-49.000000
+1 1:-42.000000
I want to delete the last 3 rows. How to do that in bash file? So far I
just know about 'sed' deleting lines, not rows.
Thank you for the answer