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Sometimes is a bit difficult depending on the situation you are in.

I will give some hints if your situation is that of really trying to be thankful for the ones you worked with.

You can begin like:

"Sometimes three years can be a long time for any of us. But in my situation three years had pass and I feel they just blew away..... I had very good experiences here, got to know many of you and the friendship and acquaintances I have here will always be in my best memories"

In a departure speech is difficult to begin, so maybe this could be a good beginning for the speech, after that just mention experiences, funny ones and what you found when you arrive there.

Be honest, simple and most of all, don't make it to sentimental, make people smile of your memories and make people feel that you really appreciate them.

Cheers and good luck!