I would have: Queen II, Innuendo, and... and... Live At The Bowl. That would be the fullest experience of Queen you could get in 3 discs.
There's no way I'm gonna buy the Greatest Hits boxset of 3CDs, because there's no March Of The Black Queen nor Ride The Wild Wind.
Ah...I've forgotten about LIVE KILLERS with the abundance of YouTube stuff online. That would really work as a great option 3 without selecting either shortened greatest hits mixes and without missing many strong "deep cuts"...smart "decisioning"
ANATO and ADATR because they've got my favourite ragtime themed songs on. And also the epic ones. They just make me happy; I consider the albums sisters. I was already a Queen fan before them but I really got obsessed after listening to them all the way through.
Now for the third one, it's a toss up between Sheer Heart Attack and Queen II. I'd probably go for Queen II.