Do indexed 3x downtube/barend front mech shifters exist?
The front mech shifters were generally not indexed.
The front gear change is agricultural compared to the rear derailleur. With so many gears (up to 13 now) so close together, getting the shifting right is critical, but on the front, the gear spacing hasn't changed forever, so the degree of accuracy is not the same.
Basically it's still in the days of change and trim until it is quiet