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THat is a 3 rather than and . It is a 3 amp fuse. Most hvac parts houses or auto parts store will have it
If fuse is blown you need to determine why before replacing or it will blow again. Have you replaced thermostat or changed any low voltage wiring recently?
OK, What happened was the wires were either put on wrong terminals r allowed to touch together and short the fuse. If wired correctly now all you need to di is grab a 3 amp fuse from auto parts, hvac supply house or place like Walmart and replace it.