Is that something new on QANTAS? I had a row of 3 seats to myself several years ago on QF and nobody said anything when I put the armrests up and laid down.

As others have noted whether the airline lets you select 3 seats together and keeps it that way is an open question.

Depending on the route a C class ticket may not be that much more than 3 Y tickets would cost you. And it's certainly much more comfortable to be in a single lie-flat seat than to try laying across three coach seats. Your C class "bed" will be quite a bit longer. So unless you are very short it won't be so comfy. Certainly it would be better than 1 coach seat.