There's NO way a 600 watt 'block heater' will replace 3 glowplugs to get her running. Maybe, just maybe IF it's stored in a heated garage(say 80*F). Had a 1200 watter on my forklift,almost ok on cool days, useless when Winter came though.
The very best a block heater does is warm up the coolant, oil and battery , to help spin the engine, does zip to HEAT the cylinders to 'diesel firing temperature'. depending on temperature, even A POWERFUL ( 1500 WATT) block heater will not help. The engine NEEDS 3 working gloplugs.