David Johnson is probably the least known of all three, at least in Baltimore. Davey managed the O's and Dave is currently a radio & TV broadcaster in Baltimore, while David hasn't had an active role in the O's family since his playing days.
David pitched for the O's in 1974-75 and Twins in 1977-78. All told, he pitched 53 games over those four seasons and went 4-10 with a 4.64 ERA in his MLB career.
One thing that caught my eye when I sent my card to David is that he lives in Abilene, TX, the town where my grandma lived for a few years when I was younger. It was nice town as far as I remember. Now isn't that interesting?