So, if nothing has changed, but something has changed it must be magic – or DHCP (which some would argue is the same thing). But seriously, from what you've described it sounds like the Pi's IP address may have changed and there is a machine (hence the
connection refused rather than a timeout) at the Pi's old address that is not running
The reason that this has happened may be related to the three flashes of the ACT LED – according to @Dmitry Grigoryev's answer to What do system LEDs signify? the three flashes means that
loader.bin wasn't found – meaning that the SD card may have been corrupted.
UPDATE: based on the OP's answer, stating that the SD card had been corrupted and that the flashes were faster than the Pi's diagnostic flashes, it is possible that the Pi was able to boot normally (and that what appeared to be three flashes of the ACT light were normal card access flashes during boot). However due to the corruption of the memory card the ssh daemon was not running and thus the connection was refused. This would be the likely explanation if you know that there wasn't another device responding the the ping. You could check that with
arp -a which would let you see the MAC address of the device at the IP addresess responding to the ping.