On my iPhone, I have text replacement set up such that if I type three letters in combination it replaces it with a predetermined string of characters. How do I set AHK to replace the letters ABC when pressed in sequence with "hello there"?
You're likely looking for Hotstrings:
Although hotstrings are mainly used to expand abbreviations as you type them (auto-replace), they can also be used to launch any scripted action. In this respect, they are similar to hotkeys except that they are typically composed of more than one character (that is, a string).
To define a hotstring, enclose the triggering abbreviation between pairs of colons as in this example:
::btw::by the way
Unless the asterisk option (*) is in effect, you must type an ending character after a hotstring's abbreviation to trigger it.
You likely would want the asterisk option: