ummm...there really isn't much strategy options in 3-man vball. Most teams play with one guy at net who blocks/sets and 2 guys back to cover the court. If you can find a big guy who can set well then you'll have a big advantage over most teams.
You can run a 2 man block scheme with each guy taking a side of the net. When the set is to the opposite side that blocker drops off take away the hard cross, cut, and dink shots. Usually the blocker has setting responsibilities but you do have some leeway to mix things up if you can pass well enough. Transitions from defense to offense will generally be trickier and sometimes will suffer from confusion on defending middle attacks/dinks.
I'm assuming you're playing in the sand and won't have to worry about too many quick & combo attacks, right?
the coffin corner? which side are you getting beat on?
Deep angle or line?
Those are tough to defend unles you have a quick guy in back that can recognize the guy changing speeds during his approach.