Does anyone have any idea which of the 3 are easiest on your machine?
I need to be able to add basic text to preexisting templates on another, slower machine and I'm wondering which will take less of a toll on it.
Alternatively, if you know of other software that could provide text functionality that would be great.
The system requirements are similar for both InDesign and Photoshop are similar (InDesign system requirements, Photoshop system requirements). Depending on which version is required for your templates, you might want to download older versions of whichever software you were wanting to use (if you're a Creative Cloud subscriber, you have access to download previous versions through the Creative Cloud app). Older apps should require less computing power.
The templates are what file type?
That will determine which software you should use.