balance is gonna be very important so you dont waver and possibly break your hand, also mainly finger + wrist strength for the power. I'd practise lots of knuckle/full finger pushups and those wrist-based dumbell curls
for horizontal, you can get 'kung fu finger strengtheners' or some shit, basically it's a square frame device with 5 things you hook your fingers onto, attached to springs so you gotta stretch them in.
http://www.martialartshop.co.uk/mart...iningother.htm
'eagle gripper' is down there, think the same equipment has about 5 diff names though in different shops.
thats just an idea though, I'm sure other people can come up with better exercise to work towards 3 finger pushup.