Things started to go upside down for Marseille once again these days. They lost their last two games. Maybe those games weren't the most comfortable ones but I would still expect to see better performances than that. 0 goal against PSG and Rennes combined... They couldn't score against PSG home while their opponent was playing with 10 players either.

These results caused the gap with the fourth position to reach seven points. There are Nice and Lens between as well so Marseille are just making things really difficult for themselves by that. While there are seven weeks left I started to think Marseille won't be making it to the Champions League.
I doubt if Marseille will be able to qualify for champions League next season because like you said they have three clubs to battle against which is Lille, Nice and Lens and the probability of Marseille wining any of them is very difficult so that's why I believe that there is no way they can qualify to the champions League, however even if we talk about points the three above mentioned clubs are far higher than them in terms of points.
However this is the time Lille needs to work more harder so that they would move themselves from the fourth position which is the champions League qualification to the third position of the league, although As Monaco will try all there best to maintain the third place but with the consistency of Lille it could be possible for them to achieve, meanwhile there upcoming match it could actually be a win match for them because they are performing more better than Marseille, so if they eventually wins the match they will have a very close points with As Monaco.