When you select the three engine mount on the Modular tanks, the centerline is offset to one side for some reason, causing thrust to not come from the center.

It looks like you copied the 4-way mount and chopped off the 4th node?

I'm not encountering any problem with the node locations during testing.

RCSBuildAid is telling me there is zero torque:

Nor am I encountering any torque in flight:

Thry the different sizes. The large version is the only one that works correctly, the rest all get offset

Noted, will try the other sizes.

In the future, it is handy to include that information in the initial report; less wasted time on testing things that don't need to be tested = happier developer = more real work getting done.

I have tested every tri-coupler adapter and could not see any off-center thrust problems.

If you have a stock+SSTU craft that replicates the problem, please feel free to post/upload/link it here and I will re-open and investigate further.