What fictional detective played by Tom Selleck had crossover adventures with the stars of both Simon and Murder She Wrote?

The fictional detective you're thinking of is Thomas Magnum, played by Tom Selleck in the TV series Magnum, P.I.

Here's how the crossover happened:

* Simon & Simon: Magnum and the Simon brothers, Rick and A.J., teamed up in a two-part episode of "Simon & Simon" called "Magnum, P.I. - Part 1" and "Magnum, P.I. - Part 2" (1985).

* Murder, She Wrote: Magnum and Jessica Fletcher (Angela Lansbury) joined forces in a two-part episode of "Murder, She Wrote" called "Magnum, P.I." (1987).

So, Magnum had a crossover adventure with both the Simon brothers and Jessica Fletcher.