RD-3500 is a "road" 9-speed derailleur. Shimano "road" derailleurs up to 10 speeds (exception: GRX, Tiagra 4700, 10-speed) used the same cable pull ratio that "MTB" derailleurs used up to 9 speed. Thus, the rear shifting is matched.
However, the front shifting isn't matched. "road triple" and "MTB" front derailleurs don't have the same amount of cable pull. Thus, it's impossible to get shifting right. With a double setup (2 chainrings) having too much chain pull in the shifting lever would be correctable (just set the limit stops right), and even very little too little pull might be tolerable too. But with a triple setup, you need exactly the correct amount of chain pull per setup.
Your options are to:
- Find a "MTB" front derailleur
- (Maybe this could work, not sure) Configure the system such that the smallest chainring is never used and you only use the middle and big rings, by setting the limit stop screw to prevent small ring from being used. Then you maybe could have tolerable shifting between those two rings.
So it isn't "double" vs "triple". It's "road" vs "MTB". But anyway, I would recommend a real "triple" front derailleur since the "double" front derailleurs don't work as well with "triple" chainring systems.