Move lines up from multiline textbox1.tex, if textbox1.text starts with ...c#

Before executing the code
First line
Second line
Third line
some text
Fifth line
Sixth line
Seventh line
some different text
ninth line
tenth line
eleventh line
some bla bla

After executing the code
First line
Second line
Third linesome text
Fifth line
Sixth line
Seventh linesome different text
ninth line
tenth line
eleventh linesome bla bla

//your lines
string[] lines = {"First line", "Second line", "Third line",
	"some text", "Fifth line", "Sixth line", "Seventh line", "some different text",
	"ninth line", "tenth line", "eleventh line", "some bla bla"};
//find lines which does not contain "line" word
var notnumberedlines = lines
	.Where(x=>!x.Contains("line"))
	.Select((x,i) => new {Index = ((i+1)*3)-1, Line=x})
	.ToList();
//remove these lines
var cleanlines = lines.Except(notnumberedlines.Select(x=>x.Line));
//add lines with "some text", "some different text", "somebla bla"
//use left outer join on index
var together = from c in cleanlines.Select((x,i)=> new{Index = i, Line = x})
		join n in notnumberedlines on c.Index equals n.Index into g
		select c.Line +  g.Select(y=>y.Line).SingleOrDefault();

//generate 50 lines: line1, line2, ... , line50
string[] lines = Enumerable.Range(1,50).Select(x=> string.Format("Line{0}",x)).ToArray();

//get every 3. line
var everythirdline = lines
	.Where((x,y)=>(y+1)%3==0)
	.Select((x,i) => new {Index = ((i+1)*2)-1, Line=x})
	.ToList();

//remove these lines
var cleanlines = lines.Except(everythirdline.Select(x=>x.Line));

//use left outer join on index
var together = (from c in cleanlines.Select((x,i)=> new{Index = i, Line = x})
		join n in everythirdline on c.Index equals n.Index into g
		select c.Line +  g.Select(y=>y.Line).SingleOrDefault())
                .ToList();

Line1 
Line2Line3 
Line4 
Line5Line6 
Line7 
Line8Line9 
Line10 
Line11Line12 
Line13 
Line14Line15 

Quote:

I would like to move every third line in the textBox1.Text on the end of the line that is above that line.

string[] splitLines = textBox1.Text.Split(new char[]
{   
'\r', '\n'
}, StringSplitOptions.RemoveEmptyEntries);

StringBuilder sb = new StringBuilder();

textBox1.Clear();

for (int i = 0; i < splitLines.Length; i++)
{
    sb.Append(splitLines[i]);
    
    if (i == 0 || i % 3 != 2)
    {
        sb.Append("\r\n");
    }
}

textBox1.Text = sb.ToString();