What to do? What to do?

Like you I have a 12-40 f/2.8, 40-150mm f/2.8 and 100-400mm.  I also have a couple of small primes and a 9-18mm wide angle.

I do wildlife photography, travel photography and have recently started doing some sports photography. I do most of my photography on trips/vacations, but also do a certain amount of general purpose shooting. For major wildlife trips I take a backpack with most of my lenses and a backup body. For shorter trips I take a small shoulder bag with the 12-40mm plus a couple of small lenses or a flash. That left me with a problem if I needed something longer than 40mm for a short trip.

I considered the 12-100 long and hard when it came out and, although it would be perfect for some shoots, I couldn't justify the cost and I didn't want to carry the extra weight (over the 12-40} on trips. The 40-150mm f/2.8 is too important a lens for my low light wildlife and sports and the 12-100 wouldn't be an adequate replacement. So, for me, the 12-100mm wouldn't replace any of my existing lenses.

I therefore bought a used Panasonic 45-150mm as a travel lens for non-wildlife trips. It was extremely cheap, is very light and gives me coverage for the occasional shot where the 12-40mm isn't long enough.

If I was starting now from scratch my decision might be a different one and I might start with the 12-100mm as my primary, general purpose lens and build my lens collection round it.