You really haven't offered enough information about the components.
If Shimano or compatible you need a Shimano triple front derailleur of 7, 9, or 9-speed vintage. The chain needs to be 9-speed, preferably Shimano. The left shifter needs to be a Shimano triple compatible shifter.
Set the low-limit screw with the chain on the largest cassette cog and smallest chainring and with the derailleur cable detached.
Flip the smaller shift lever several times while pulling all slack out of the cable.
Then attach the cable to the front derailleur being very careful with the cable routing.
You should need very little if any cable tension to make it shift properly.
Shimano triple shifters have four (4) main positions plus a trim position. This effectively gives you two positions to use for trimming each chainring.
Al