p wrote:
But 1.5 mm by two is a 3mm.
No. The measurement is the diameter of the core, but the current
carrying capacity is related to the cross-sectional area.

Two by 1.5mm has an area of 3.53 mm2

One by 2.5 has an area of 4.91 mm2.

Isn't this combination supposed to carry a
better load than a 2.5mm.

No. You would need three 1.5mm cores to carry more than a 2.5mm core -
and that assumes you can get all three cores to carry their fair share
of current. That is a pretty poor assumption in the long run (which is
almost certainly why parallel conductors are essentially banned in the
regs).