Please check the polarity on both the output and bias windings - the output and bias windings  should  go negative at the anode of the output or bias rectifier when the primary switch is conducting. If the windings are configured incorrectly, the output will collapse when even a small load is applied. The polarity dots on your transformer appear to be correct for a flyback converter, but the actual transformer may be configured incorrectly. What is your output power?  Depending on your intended output power, a lower current single rectifier may be more appropriate. What is your intended output power, by the way?

Did you check the transformer in you intended design using PIXLS to make sure that it is appropriately designed for your  intended application? To do this you will need to get information from the transformer vendor regarding primary, secondary, and bias turns, type of core used, and primary inductance.

BTW, I just checked the transformer data sheet, and it says that for 12V, 3A output, you should be connecting output windings S2 and S3 in parallel, so pin 6 should connect to pin 9, and pin 7 should connect to pin 10. That way, you can use a single output rectifier. 

Hi and thank you for your reply,

I just found the issue, it was the TL431, I don't know why, but when I replaced it with a simple 10V zener diode, it is working correctly.

I used the recommended design in the datasheet.

I just had a closer look at your schematic - R6 at 1k is too high a value, and will strangle the amount of current that the optocoupler transistor output  can deliver to the TOPSwitch  control pin. Something like 150 ohms is a more appropriate value. Also, add a 1K resistor across the optocoupler LED so that the TL431 can get its required bias current. That way, you should be able to get the circuit to work with the TL431 instead of a zener diode.

The transformer you are using appears to be more appropriate for a 20W design. I have attached a spreadsheet that shows a design using an EE25 core (similar parameters to an EI25), which is the core that your selected transformer appears to use.  I have also attached a report on a standard 30W design using  TOP245 and an EF25 transformer. Was your TL431 pinned out correctly?  Looking at the flat side of the TL431, the pinout should be (from left to right) reference, anode, cathode.