Read some of snYpir's tutorials, particularly
Briefing.html tutorial
How to use objectives and init.sqs
OFPEC Guide: MP Missions
You should also at least skim all of the tutorials in the Getting Started section.
Group names and unit names are not interchangeable. (There are a very few commands where you can use either.) Groups and units are profoundly different things.
Alive is only a useful command for soldiers: although it does technically work on vehicles, it is not reliable and you should use canFire and canMove commands instead.
Lines you mind find useful, syntax not guaranteed:-
"alive _x" count units grp1 < 1
(not canMove tank1) and (not canFire tank1) and ...
("canMove _x" count [tank1, tank2, tank3] < 1) and ("canFire _x" count [tank1, tank2, tank3] < 1)
When you are finished making the mission, upload it to the Missions Beta Testing board. Read the sticky topic on that board first.