In Temasek Polytechnic, Singapore, several AI chatbots acting as digital teaching assistants were trialed between July 2021 and August 2022. As these were developed for different purposes, it is important to learn if these AI chatbots help achieve the desired learning outcomes. This paper focuses on lessons learnt from three chatbots designed for different learning experiences: the independent learning of EXCEL, the correction of errors in the application of APA citation style, and the provision of supplementary theoretical support for software usage for Social Media Analytics. The research team studied the students' reaction to these educational chatbots, their motivation to use them, as well as the efficacy of the chatbot features to support specific purposes. This paper offers a design model that describes the three-stage approach for learning with an AI chatbot, with examples elucidating them. These sequential stages are foundational in the chatbot design to provide focused, personalised, student-controlled online learning environment. The application of this model is detailed in a fourth AI chatbot designed for the subject, Airline Management.