Alignment applies to a line of text and you cant mix alignment on a line. I have got around this in MS Word, but should also work in Writer.
Place 3 textboxes on the line, one at the left margin, one at the right margin, and one between these. You can set the text alignment to left for the left textbox, centre for the middle textbox, and left for the left textbox. Set the width of the left and right textboxes to be the same and set the middle text box to be exactly between them. Hint - set to show a line around the textbox to easily set the correct textbox positions.
On second thoughts use a 3 column 1 line table as its easier to line it up. Again you can set the text alignment for each of the 3 cells as you want to.