Since Reiki has been passed down via different Reiki Masters and Reiki Teachers (Eastern and Western), you will find there are variations in what is covered in Reiki Three (both in substance and in form). However, the main teachings should be covered. With the attunements (initiation, empowerment), you are "connected" to the Reiki Source, enabling you to become a channel for Reiki Energy.
Reiki Level 3 Course (also known as Reiki Third Degree / Shinpiden)
The third degree in Reiki is for Reiki Practitioners who, having completed the second degree, wish to become Reiki Masters and Teachers. Most Reiki 3 courses or workshops cover the following (not necessarily in this specific order):
Quick recap of Reiki history, hand positions, principles and ethics, Symbols from previous learning, etc. (where necessary). Many Reiki Masters and Teachers expect you to wait for 6 to 12 months after completing your Reiki Second Degree before they accept you on a Reiki Master Teacher Course.
Some Master Teachers will allow some students to have a shorter gap between Reiki Second and Third Degrees.
Reiki III teaches you how to become a Master and Teacher. Completing the course is just the beginning ... your real Reiki journey begins as you learn to master Reiki Energy on a daily basis. When you then start teaching with an open mind, you also become a better Master.
Note: some teachers offer the Third Degree as 3A (Master Course) and 3B (Teacher Course). You will find in some versions there are even more layers and levels. Take a course that teaches you what you need to know to become a Master and Teacher.
Meditation / Relaxation
Master Attunement (empowerments)
Learning and applying The Reiki Master Symbol (some teach one or two more Symbols); also working with the earlier Reiki Symbols from your Reiki 2 course.
Meditation and exercises
Healing methods, techniques, ethics
How to plan / teach Reiki 1, 2, 3
How to pass attunements and empowerments
Practice / exercises
Finding students, how to run your course, finding your own teaching path (since there are many versions)
Reiki lineage - a line of how Reiki has been passed down from Dr Mikao Usui to your current Master and Teacher. Once qualified, that becomes your lineage with your name added after your Master and Teacher's name.
Further personal development
Feedback, questions and answers
Reiki 1 / Reiki 2 / Reiki 3