Unless you've got a 15-0-15V >60VA transformer + the 30 to 48V in, 5A@5V out DC-DC converter hanging about essentially free, it probably doesn't make much sense to do it that way. If the transformer is run too close to its rating, the 7815 and 7915 regulators will be prone to dropout at the ripple troughs if the 5V rail is heavily loaded, so I suspect a 50VA transformer would be marginal.
The easy low cost option would be a 19V small laptop PSU, 7815 for a clean +15V rail, buck converter for the +5V rail and inverting switching regulator to get a -18V feed to a 7915 for a clean -15V rail.