Connecting three tables: client, year, course(s)

I run a yoga-studio, and would like to use Base as a kind of CRM, registering my clients purchase history (using data from my webshop). On a clients record form, I would like to pick a year, say 2013, and then have a dropdown-list over courses that were available that year. And then choose from that list to register the clients course(s) in 2013. The same would apply for any other year, of course, until I have a complete history in Base.
I have figured out how to establish relationship between two tables, for instance «clients» and «courses». And, of course, the same would appy for tables «years» and «courses».
But I haven’t figured out how to connect three tables, so that I can enter purchase history like mentioned above (client→year→course(s) for that year). I’m sure there’s a simple solution to this, and would be grateful for any assistance.
If my description is unclear, let me know, and I’ll try to clarify.

See if this may work for you. I see three tables. First table is ‘Courses’ containing ID (maybe a unique # which can be used as key), Year and/dates and other pertinent information. Second table is ‘Clients’ with ID, and other data (name, address etc). Final table is ‘Purchases’. This table has an auto-incrementing ID for a key, Client_ID (ties back as one-to-many from to ‘Client’ table) and Courses_ID (again ties back to ‘Courses’ table).

With tables in place, the other comment in your question regards forms. Using filtering (see this post) you should be able to create/update/list records in most any combination. In that post are many samples which should help.

Thanks, this seems very helpful. And I’ll look into the post you mentioned.