The CRC lists the mp of K3PO4 as 1340 °C without dec. But it is hygroscopic! Cooling after heating in an open crucible will cause it to reabsorb moisture.
You need to put it into a Schlenk tube and heat it on on a high vac line at say 150 °C (silicone oil bath) overnight and dry to const weight (check IR for O-H).
And keep your dried product under N2. You might even get away with putting the sample under high vac at RT in a desiccator over P2O5. Good luck!