ECU light won't stop blinking

the check engine light came on , i turned the engine off , turned the key on, and it blink every 3 seconds, what does that mean?

How many blinks does it blink before the 3 sec pause? if its just one then that is an oxygen sensor code. You can find a list of the codes in the faq at the top of this forum.

it is a constant 3 second -blink/pause/blink/pause/blink...............

how fast does it blink when it sending codes with more than one blink

Is it suppose to keep resending the codes the whole time the ignition is on or the engine is running?

Whole time the ignition is on, or whenever you develop a code.

When emitting a code itll be like "blink, blinkblink, blinkblinkblink.......repeat." This would be code 1, code 2, and code 3.