If it blinks for 3 seconds and turns solid it means it didn't work. It would blink for ~60 seconds and then shut off, also if your USB flash drive has an activity light, that would be blinking as well for the same duration.
Recently got my hands on a known-broken P8Z77-V PRO.
Powers on, fan spins up, CPU LED flashes, and then DRAM illuminates and stays solid red. No POST.
Tried 2 variations of DDR3 in all slots to no avail.
Tried a 3770 and 2600k, nothing.
Was seemingly able to flash the BIOS using the BIOS...