That is exactly what I did!
Sorry!
I disabled the editor and put the line, as you can see above, in the HTML code!
Sorry if that is bothering you.

do not bother anymore.
I put in the custom.css
h3{
text-align:center;
overflow:hidden;
}
h3 span{
display:inline-block;
position:relative;
}
h3 span:after, h3 span:before{
content:" ";
display:block;
height:1px;
width:1000px;
background:black;
position:absolute;
top:50%;
}
h3 span:before{
left:-1010px;
}
h3 span:after{
right:-1010px;
}

now it works. Thanks!

Problem solved.