I am likely an ISTP 9w8 but I am positive that I have 3w4 in my tritype, and I relate to it quite a bit. Since I'm not a core 3, and also because I don't think that I am very knowledgeable with enneagram, I'm not the best person to be posting here. But I truly think that Type 3 and ISTP are not contradictory, although uncommon.
You've posted some stereotypes about ISTPs. To be fair, I can't say that I've ever met any who aren't laid-back or go-with-the-flow. Even if we're secretly not, we do give that impression. But one thing I noticed about ISTPs is that many tend to be lazy with a lot of things, EXCEPT for a few things that we really care about. This is true for me. Check out this linked thread for example, where some of us have admitted to being serious perfectionists at times. If motivated by a desire for achievement/success, this sort of behavior can certainly be Type 3.
Personally, I don't really hate planning as much as I feel that I am bad at it, which makes me want to avoid it. I'm fine doing it when necessary. Maybe other ISTPs feel that way? And I think a lot would just be apathetic to planning rather than hating it. Also, while I prefer to think in the moment, I can't help sometimes but to think of my future, and not much scares me more than living an unsuccessful life, which motivates me to do stuff.