Kind of a broad question. Learning what matchups are good for specific comps you're running is a good place to start, or posting teams you have trouble fighting. Depending on your roster, you may want to also consider loading up on 2 teams, and punting your 3rd. 

Let's start off by posting your roster, and where you're at in 3v3 too. 

In my mind you want to generally approach 3v3 a bit similar to 1v1 (or normal) arena.  How are you progressing there?  Do you have a few good teams to tackle the various ones that come up?  For example, speed nuke teams that target slower teams, or go second teams to target speed teams, or a buff striping team to target the unkillable ones, etc.

Once you start thinking about individual teams, you can set them up similar in 3v3 content

So where are you up to in normal arena?  Silver or Gold?