If using a steel frame for a conversion to 3 speed or other IGH the dropouts can almost always be cold set to the required width. IGH widths have varied from about 110mm to 135mm depending on make, model and manufacturin era. The main consideration in converting a steel frame is IMO to find one with semi horizontal dropouts which generally means older frames. Frames with rear opening track style dropouts are also good. Vertical dropout frames usually require a chain tensioner be used which IMO just does not look right and is added complexity and expense.

If using an aluminum or CF track style frame then you need to find an IGH of the same OLD as the dropout width or quite close to the same.