A typical electric horn does not have much amperage draw. I have used 3 horns in place of the original 2 on a number of American cars without difficulty.
The horn circuits I have come across all seem to be protected by a 10 amp fuse, which is above the power draw of 3 horns, so imo you likely could use 3 horns without difficulty.
As for wiring, I would suggest just use the positive lead from the stock wiring harness and make your own jumper wire to connect the 3 horn terminals. Try using the chassis ground as well, you may not need to use the ground wire at all.