The only reason I bought BTT is cuz I just want to change the action of the 3 finger swipe from whole windows to tabs.
How do I change MacBook trackpad gesture from 3-finger swipe for whole screen windows to 3-finger swipe to tabs (like chrome tabs, or safari tabs)?
Go to System Preferences → Trackpad → More Gestures and change the "Swipe between Full screen apps" to four fingers. Or turn it off if you want some personalized action with BTT for that.
Inside BTT, create a "Conditional Activation Group" (see Screenshot) and set it up with "Any of the following true" and always "App Name" + "is" + "Your exact App name".
Create a four finger swipe and give it as association "Send keyboard shortcut" ⌃⇥ (for next tab). The second four finger slide should get a ⌃⇧⇥ action. Too lazy to do it twice, here's what the first action should look like.
This presumes that all apps you want to use it for support this keyboard shortcut to switch tabs. For those which don't you'll have to create an App Specific Action setting the appropriate Shortcut.
In order to create a new gesture to swipe through fullscreen (deactivated in Step 1.) make sure you create it in the "All Apps" section of BTT, not inside the Conditional Activation Group
Hope this helps