The war began in late 1534 when a landlocked Toungoo led by Tabinshwehti and his deputy Bayinnaung, tried to break out of its increasingly narrow zone by launching a preemptive war against a weakly led Hanthawaddy. In the beginning, Toungoo's maneuvers amounted to a mere raids of Hanthawaddy territory, and its initial dry-season raids in 1534-1535, 1535-1536, and 1536-1537 all failed against Pegu's fortified defenses aided by foreign mercenaries and firearms. In each campaign, Toungoo armies had only 6000 to 7000 men, a few hundred cavalry and a few dozen war elephants and did not yet have access to foreign troops and firearms. Unlike his father Binnya Ran II, King Takayutpi of Hanthawaddy could not organize any retaliatory action. His nominal subordinates in the Irrawaddy delta and Martaban did not send any help. Nonetheless, Pegu's defenses led by two leading ministers of the court, Binnya Law and Binnya Kyan withstood the raids.
Question: How many years did Toungoo's dry-season raids last?
Answer: 3