The SRC_URI for dev-libs/mpfr-3.0.0_p3 provided in the ebuild isn't valid anymore. Reproducible: Always New download locations seem to be at http://mpfr.loria.fr/ I made a local ebuild with SRC_URI="http://mpfr.loria.fr/mpfr-3.0.0/${MY_P}.tar.bz2" and fetching seems to work fine with this.
mpfr.org is still the main site http://sources.gentoo.org/dev-libs/mpfr/mpfr-2.4.2_p3.ebuild?r1=1.8&r2=1.9 http://sources.gentoo.org/dev-libs/mpfr/mpfr-3.0.0_p3.ebuild?r1=1.10&r2=1.11 http://sources.gentoo.org/dev-libs/mpfr/mpfr-3.0.1.ebuild?r1=1.1&r2=1.2