UCC28600: Using identical transformers in parallel for 300W design

Hello YaD, 

Thank you for your interest in the UCC28600 flyback controller. 

I can't open your file because I currently don't have access to the TI-TINA program. If you can kindly post a pdf of the simulator schematic or post a high-resolution screen image of it, I can still review it that way. 

Having said that, if you have simply paralleled all primary windings together and all secondaries together, this arrangement will not work.  It may actually simulate properly since all of the parameters will be identical, but won't work well in real life because small impedance differences between windings will result in massive current unbalances. 
Dynamic switching voltage transitions will drive resonant currents ringing between the windings that will not be measurable outside of the paralleled windings. 
This will result in excessive I^2*R losses in both primaries and secondaries, with unpredictable current distribution sloshing between the three transformers' primary windings during demagnetization.   

On the other hand, it is possible to connect "identical" transformers in parallel if all the primaries are connected in series, and all the secondaries are in parallel. 
This works because the magnetizing current is forced to be the same in each xfmr during the ON-time, and (aside from very minor differences due to inductance variability) the energy stored in each core is substantially the same and released to the output during demagnetization.  It doesn't matter which core runs out of energy first or last. 

For the series-connected primary arrangement, the new primary inductance of each xfmr must be 1/9 the value of the original so that 1/3 of Vbulk across each will generate 3x Ipk of the original to match the same expected total 3*Ipk of the original paralleled primaries.  Also, the primary-to-secondary turns ratio of the new xfmr must be 1/3 of the original, so that each output current will start at the same secondary peak amplitude as in the original concept.  

Of course, this reply is based on an assumption of your original paralleling concept without having seen it.  If it matches and makes sense, then we're done. 
If my assumption is incorrect, then I'll wait until I can see your schematic and revise my advice accordingly. 

Regards,
Ulrich