I went to Star Archive.com to look for TC cast members addresses, and found them for Joyce DeWitt, John Ritter, Suzanne Somers, Richard Kline, and Don Knotts, among others.
Don Knotts and Suzanne Somers have a reputation for not responding, John Ritter is late (But he sends good pictures), and that Joyce DeWitt has almost always responded. Now I have a question: Some celebrities make you pay for these things. Do John Ritter and Joyce DeWitt do the same thing?

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Jack: You know what your trouble is, Chrissy? You're a compulsive eater!
Chrissy: I am not! I'm just always hungry.
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Creator and Co-Writer of "Four's A Couple", Writer and Creator of "Who's Cooking With Lana?","Snow", and "Three's Company Too!".
Find these in the Three's Company Fan Fiction Board!

Well when John was in Broadway from Oct 2000 to June 2001, he got back to people lightning quick! 1-2 weeks max!

But I also tried the Ventua address in California, he got back to me in a few months...but the Broadway one was like 8 days!