I started long Eng flash cards (because I work late, so I ask my maid to flash) to my girl when she was 3 yrs 8 months, basically she can read 90% of the 25 flash cards in the "pool" after flashing for 5 days. I really do not think it is late to start at 3 because it works, but for sure, starting earlier is much better.
Besides the hand made Eng cards, I also ask Jimmy to print 32cm X 32 cm picture cards with Eng words on them to my girl (now 4 years old) and my son (26 months). We have a small mother group in our office for the flash cards (totally 6 moms), and we exchange experience in the flashing. That gives us mutual supports amongst ourselves (to continue this tough task!).