I have a DC6V relay JQC-3FC(T73) and wants a approx 3 second relay when it turns off.
OK what is driving this, an Arduino?
If so just delay for three seconds before you turn it off.
If no micro is involved use a monostable driven by your turn off signal. Feed the output of the monostable into a flip flop which controls the relay. Make this flip flop edge triggered to trigger off the end of time edge of your monostable. To turn it on simply trigger the other flip flop input directly.
if it is just delay that you require then within arduino there is a function called the delay(). delay (3000) should delay the relay for 3 seconds once energized. if its a 6 volt relay then you need to power it externaly and use a NPN transistor to switch it on.
Why not get a 5v relay that is compatible with an arduino ? its quite cheap too.