Correcting the chainline on triple crankset with chainring spacers I have Marin Four Corners bike with 3x9 Sora crankset (most likely FC-R3030) for which chainline is not given on Shimano site, but I've read all road cranksets have 45 mm chainline.

I would like to replace it with 3x9 Acera (FC-MT300-3) which has 50 mm chainline.

Since I would need to narrow down the chainline I am thinking about adding 5 mm spacer to biggest ring which should shift all the rings towards the frame. However this method is listed on Sheldown Brown as suitable only for single chainrings. Is it a simple omission or it is indeed not possible with 3 rings?

To avoid "whys" -- I need low gears with smooth shifting (ratio close to 1.1), this means tight cassette like 12-26 and small crankset.
When 135mm-spaced QR and 142x12 thru axle disc road frames became the norm, a lot of what was written about bike chainline took on a hidden caveat that in many cases hasn't been corrected, which is that those bikes put the rear ideal chainline in exactly the same spot as their mountain counterparts. Many higher end road cranks are now offered in two versions (disc vs rim brake) for this reason.

It's confusing because the industry has responded slowly to this and a lot of road cranks do still only come in one version despite getting put on both 130 and 135mm bikes.

You actually need a chainline of about 47.5mm. You can corroborate if you want by taking your back wheel off, measuring with a caliper or pocket ruler with one eye closed the distance from the centerline of the 5th cog to the face of the right axle locknut, and subtracting that measurement from half of the hub's over-locknut-dimension aka spacing.

50mm front chainline is fine with a rear of 47.5. That 2.5mm in either direction approximately corresponds to how much you can push the chainline without any major consequences, although this stops being true for very short chainstay bikes.

You can't adjust the chainline of most triples with spacers because you can't use spacers to shift all the rings in the same direction. There are some crank designs that are exceptions, but not this one.

For square taper and ISIS/Octalink you can adjust the chainline with different length BB spindles.