Disney have a trend of ending shows after 3 seasons and they has been no annoucement regarding a 4th season (which to be fair is normal)
The 2010's disney shows
Phineas and Ferb lasted 4 seasons (technically started in 2009 but was a huge show for them and mostly aired in the 2010s)
Gravity falls was 2
Milo Murphy was 2
star vs the forces of evil was 4
Wander over Yander was 2
Penn zero was 2
Tron uprising was 1
Jake and the never land pirates was 4
sofia the first was 4
mickey mouse was 5
the 7d was 2
Kick Buttowski was 2
Motorcity was 1
Pickle and Peanut was 2
Descendants: Wicked World was 2
Future-Worm! was 1
fish hooks (2010-2013) was 3 - was announced the third season was the last season shortly after it started to air
Two More Eggs (2015-2018, animated shorts) was 3 - was announced the third season was the last as it was renewed
The Lion Guard (2016-2019) was 3 -I'm not sure when it was announced the third season was the last
Rapunzel's Tangled Adventure (2017-2020)was 3 - Was announced the third season was the last as it was renewed
Elena of Avalor (2016-2020) was 3 - was announced only a month before the last episodes aired that it was coming to an end
the rest are still airing. The point being that while there have been some notable shows ending after 3 seasons, there are plenty of notable series that went 4+ and in general the shows that ended on the third season announced the end was coming well before the point we are at in ducktales.
I can't see any reason at all to assume that the show is ending with this season, but alas fandoms always assume the show is going to be cancelled until there is an announcement of more even if their are mounds of proof that the show was still in active production *glares towards Steven Universe fandom* Note: I haven't seen any mounds of proof that this show is still in active production, but there also hasn't been any signs that it's wrapped up production, which if this was the last season we should have seen, but Disney can also be quite good at hiding things. We actively knew when people were moving off Milo Murphy's law though well before the second season finished airing.
I will also note I don't think plot wise it makes sense for this to be the last season given the number of episodes we have left. Perhaps we will super rush it, but it feels far more likely that this season will end with some sort of significant FOWL victory rather than FOWL fully resolved, and that the 4th season will be the last season as the show slowly works back towards status quo and ends on a theme of adventure is always waiting, or perhaps even instead a movie to wrap things up, but if they were doing such a thing I'd expect them to have already announced a movie without covering what theme it will cover.