Are you going to make just one sphere? You can make tooling to
create a hemisphere with a Bonnydoon press but the cost of having it
made will not be trivial.
A dapping set is probably not going to work very well just because
you will not find one with a 3" opening. Your blank for a hemisphere
will be 2.8" in diameter and to dome it up without cutting into the
surface you will want the disk to fit into the opening in the dapping
block. And you will want several steps between the first opening and
the final 2" diameter opening. I haven't seen a dapping set in this
size just some with one or two holes and punches in that size range.
If you have both a dapping set with a 2" hemisphere and a matching
punch and a Bonnydoon press you can use a combination of contained
polyureathane blocks and the dapping set. I have a sphere I made a
few years ago that is about 1.75" using the press with the contained
blocks and a dapping set
You can raise it, hemispheres are tough but it can be done.
Your best method may be to find a metal spinner and have the
hemispheres spun. I have used Paul Wiley
http://www.paulwileyspinning.com for a project and he does good work.
You can also learn to spin and do it yourself but you will need a
lathe and some time to learn the process
James Binnion Metal Arts