I do not think that the multiplier of current DF Natsu is 3!!! Imho, it's more. You just need to take a look at Acnologia. Acno's dragon form is the result of mastering DF and his power up is huge when he's transformed. In dragon form Acnologia only needed to use sheer force and perhaps sometimes roar attacks to defeat his opponents but we never saw him trully going all out!!!
This basically means that stronger dragon-slayers who activate DF get even a bigger increase in power than weaker ones because they dragonize more and this kinda sounds logic, to me!!!
ToH DF Natsu was already on wizard saint tier while ToH Base Natsu was just a s-class mage back then. Tartaros DF Natsu was able to do some damage to Etherious Mard Geer ( probably GoI-level) who was much more powerful than everyone else in that arc.
Jellal could handle current Base Natsu and FDKM Natsu for sure and take the win here but at the moment I just see no way how Jellal could surpass DF Natsu!! However, Natsu's DF is just speculation and hype and this troubles me which one to vote

!!!
Jellal vs Base Natsu: low diff win jellal
Jellal vs FDKM Natsu: mid or high diff win Jellal
Jellal vs DF Natsu: Natsu takes this