The spindle-length is most likely not an issue because the inner ring is usually spaced really close to the chainstays, 3-5mm. Usually if you put a spindle that's shorter than the triple cranks were designed for, like a double, the inner ring would end up hitting the chainstays. So you'd know immediately if it was the wrong one.

Measure the distance between the center of the seat-tube to the middle chainring. Or you can measure the distance from the edge of the seat-tube to the middle-chainring. Then add 1/2 the diameter of the seat-tube. The distance should be around 40-45mm.

I suspect it's a FD adjustment/rotation issue. If you have the FD too high above the rings, it has to move inward and outward a lot to make each shift.

Check out the front-derailleur adjustment procedure at Park Tool. A very important distinction they do that a lot of shops miss is aligning the outer-cage parallel with the chain in the highest gear, not the chainring. This gives you a wider range of rear cogs that don't rub.