if u r using Shape for the multiple lines,,this method is useful to solve your issue by using photoshop

by selecting your multiple shape layer

  click on subtract from space area  

select polygon tool set sides to 3 drag the mouse u get the triangle shape place as per your need it will subtract the shape

You can use shapes for the lines, and subtract one of them. This method is useful to solve your issue using Photoshop:

By selecting your multiple shapes layer you can then click on subtract from the space area:

Then select the polygon tool, set sides to 3, drag the mouse to get the triangle shape in place, and subtract the shape.