Chainline issues

When my 10 year old chain snapped, I realized that I needed to replace my rear cassette and crank, due to wear. Noted for the future! I ordered a 9 speed cassette from Shimano (same brand and number of gears as I had), and a 3 speed front crank (again, same amount of gears and same brand, different type though: Shimano Altus M371). I thought I had taken all the factors into account, but of course I was wrong :). I noticed that I could not adjust my derailleur to smoothly shift gears without rubbing. I didn’t know about the concept chainline, and I think it is very likely that my new crankset has a different chain line (50 mm) than my old one (unknown).

The problem that I have is that I can’t find what chain line my old crank had. I think my options are either:

1. find out what chain line my old crank has, and try to find something similar. -> This seems very difficult, because most cranks I can order have 50mm (the same as the one I just bought)
2. play around with spacers. I am afraid this gets me into a rabbit hole. I don’t exactly know what I need and I am afraid I will ruin something when I have to dissassemble my bottom bracket.

A picture to give it a little bit more context:

Is this the moment to go to a bike shop? Or is there an easy fix that I could apply? I have a three piece crankset (I think it is Shimano Octolink).