Escaping plus signs doesn't work

Question

I'd like to delete all lines with 3 plus signs:

+++ 3 plus signs
+ 1 plus sign
+++ 3 plus signs

Here's my sed command, escaping the plus signs after beginning of line

sed '/^\+\+\+/d'    -> This erase all lines
sed '/^+++/d'       -> This works and show only the 1 plus sign line

Why? Is there any problem while escaping the plus sign?

Answer 1

"A quick comment. The original sed did not support the "+" metacharacter. GNU sed does if you use the "-r" command line option, which enables extended regular expressions."

(Source)

If you don't use sed -r, then you don't have to escape + since it is not considered a metacharacter.