I am speaking from general and not specific knowledge.
I am surprised the appliance was not supplied with a plug fitted. I assume you must have the three pin socket which you describe already fitted, which has at least a 20 amp (probably belter a 30 amp) supply.
If you removed the cover of the socket (turn of power at fuse box first) then you will be able to see the wiring and thus know which is the power, negative and earth. Thus you can connect the plug accordingly.
If in doubt get an electrician to do it.
PS From experience in Spain : Never accept that the original wiring is as it ought to be. I once found that an earth wire (green and yellow) had been used for the power supply to a socket and the earth was on the power wire. I also had an occasion where I turned off all the power to an apartment at the fuse box but the socket I was working on was still live. I never discovered where that socket was obtaining it’s supply from: Possible from next door !