Just to see if anyone did this and if it works:
I am running a 3x9sp setup. I need to change my front derailleur. Can I use a 3x10sp derailleur?
The shifter will still be a 3sp so the ratio will be fine. My only questions has to do with chain width and if it has clearance on the derailleur.
Thanks!
The cage width will be narrower and that can cause it to be finicky or impossible to adjust without rub anywhere. Usually it will be something along the lines of once the cable tension is right for it to shift smoothly, it will want to rub in either the highest gear or in mid ring, largest cog. Chainring spacing isn't strictly universal so it does depend on the crank.
Tweaking the cage can be a solution but not a guaranteed one. Just running a 10 speed chain on your 9 speed cassette is totally fine. Shifts great without any issue I've ever seen, and helps clear up any FD rub issues.
Be careful not to get snared by the minimum difference between middle and large ring FD spec. It's easy to do, particularly when putting a new FD on an older road triple. Numbers for all current Shimano derailers can be found here.
