Encrypting where multiple parties need to decrypt

Thought that some folks here might provide guidance or experience.

I have a system for a client where multiple 3rd parties need to provision encrypted files via ftp,

Here is the challenge. I need to decrypt the files to use them BUT my client also wants to be able to decrypt them.

As it is generally considered poor practice to have a shared private key, how can 3rd parties encrypt the file in a manner that both myself AND the client can decrypt them?

Thanks in advance.

You can read about secret sharing schemes. From what I think is, you can divide the secret key into shares, and anyone with the share can decrypt the file. You give the shares to only those that have access to the data. In this way, your full secret key is not shared with everyone.