Hi HudaThunkit,
It’s actually the other way around and if everything else is the same a thicker layer will feel softer than a thinner layer (see post #14 here).
If all the layers (both of the 3" layers and the 6" layer) were the same ILD (and making sure of this may be a big “if” because Dunlop has a wider firmness range than Talalay) then they could be similar in terms of firmness but it would also depend on how much the layers were compressed because they would only “match” if they were compressed by 25% of their thickness. Depending on any layers above and on your body weight and sleeping position, if the layers were compressed either more or less than this then one would likely feel firmer than the other. Dunlop gets firmer faster than Talalay with deeper compression (it has a higher compression modulus or “support factor” than Talalay). There is more about this in the first part of post #6 here and in post #6 here.
Of course if the ILD’s were not the same then the one with a higher ILD would be firmer than the other (compressed to 25%).
There would also be a difference in “feel” between them.
Phoenix