How to calculate the cable cross section 80 m long for connecting a load of 3 kW?

Calculate by voltage loss, choosing acceptable losses based on the type of appliance. In general, no more than 3-5%. Here you need 2.5 (3% loss) or 4 mm² (2% loss) for copper. Is this machine suitable? I don’t know, the load is three-phase? If yes - then the excess rating, if single-phase - also excessive. For 3 kW in most cases, 16A with a single-phase connection is sufficient. If you connect an electric motor - then look at the starting currents and time-current characteristics.

p.s. Excessive denomination does not mean that it does not fit. Just this is more than enough and it will pull up to 5 kW with a single-phase connection. It all depends on the other end of the line - if there are sockets, then put the machine on 16A.