The -3 Penalty comes from the commerce industry itself, not the dealmaker. But I think I get what you meant, how about:
I know. My suggestion was reduce the penalty from -3 to -1. Commerce seems to be balanced on the assumption that the player
will get a Dealmaker and use it. So, nerf the Dealmaker to say... +25%. That way, Commerce is more useful for those who do not use Dealmakers and make core worlds with Commerce less prone to getting 0 stability from pirate activity or other destabilizers.
Without Dealmaker, assuming I did not wipe the core worlds, I am better off building another industry on a planet or even building a sixth colony and eating a -2 mismanagement penalty on some planets instead of eating -3 stability and -1 industry slot from Commerce (since Commerce takes a slot).
+25% income from stock Commerce (combined with other drawbacks) is a joke. Up to +50% income from improvement is okay, except that gets into the 2^n madness when I want more improvements elsewhere.
P.S. I guess Dealmaker giving +50% income and lowering stability more (for classic -3 total) could work too.