Adding centre column to my header

Since your linked page does not have the centre element, I cannot tell you exactly what the problem is. One problem might be the right column is inside a 75% width element. (class col-md-9 containing id header-right) Even if its content takes up less than half that, it’s still space unavailable for other columns. You need to restyle the right column to take up less room.

That may not be enough. Some themes like to sneak in divs with clear: both styling that forces a drop in the element even if there is room on the same row.

Also be sure column containers have float: left (or right) styles or they will automatically drop below adjacent elements.

It doesn’t look like these last two are the problem here, but they are common causes of unwanted drop down behaviour.