The machine is pulling more amps than the circuit can handle, or there's a problem with your extension cord.
First, check the circuit. The SPX3000 draws 14.5 amps. Switch off any other machines (like a fridge, freezer, or shop vac) that are on the same circuit before you use the pressure washer. If the fuse still blows, the fuse itself may be too small for the amp consumption of the machine. You may need to install a fuse with a higher amp rating.
Second, ditch the extension cord. This is a huge culprit. Try the unit without an extension cord if the fuse blows. If you are using one, make sure it's heavy-duty, watertight, and as short as possible.
If that didn't work: There could be a defect in the machine's electrical system. Inspect the power cord for damage. If the cord insulation is damaged, do not use the pressure washer. Contact Sun Joe service.
Heads up: Protection should be provided by a Ground Fault Circuit Interrupter (GFCI) on the circuit or outlet you use. Many outdoor receptacles have this built-in. It's a critical safety measure for any electric pressure washer.