Current calculator
Estimates for planning and study. Fixed wiring must be designed and installed to the wiring regulations that apply where you are, by someone competent to do it.
Current is power divided by voltage, adjusted for power factor on AC. A 3,000 W load at 230 V draws 13.04 A. For a continuous load, size the protective device at 125% of that — about 16.3 A, so a 20 A device.
How to calculate current
The 125% rule exists because breakers and cables are rated for their steady thermal limit, and a load running for three hours or more sits right at that limit with nothing in reserve. Sizing at 125% gives the protective device somewhere to live. Motors complicate it further: starting current can be six or seven times the running figure for a second or two, which is why motor circuits use time-delay protection rather than simply a bigger breaker — a bigger breaker would protect the cable less while still nuisance-tripping on start.
Questions
Divide watts by volts. On AC with a power factor, divide by volts times the power factor as well.