Another reason why you should focus on protecting yourself from air, even if it delays your T3 is because air has only one counter, aa. You can't wall it out and it's very hard to run from. Mechs you can wall out, turret, and attack from your cc.
A couple of problems with this:
1) if you are building walls you are either a) wasting carbon for light walls which are completely worthless or b) wasting ore which will hurt you even more
I play a LOT of 3v3 and the typical situation goes like this:
Who is biggest threat? a)known player b)if no known player, player with highest score..
If with people you know on team (i.e. clannies) EVERYONE focuses on main threat first, trying to send in a good combo (ex: air from player 1, strike mechs from player 2 and troopers from player 3...
Are you trying to take them out? Not necessarily... main goal is to CRIPPLE them forcing them to have to rebuild...
After that is accomplished, work is split up into harrassing the other 2. Even if you've got a GOOD 3v3 with an even match up (which rarely happens) I see this type of action almost everytime.
I have also seen: Coordinated attacks with all 3 players on one team hitting the other 3 SIMULTANEOUSLY making it impossible for one player to help save someone else.
My main attack has gravitated to strike mech rush followed by air... strike mechs are harrassing workers and taking out any anti-air troopers that are lurking about. Counter to this? Easy. Put some grenadiers with your anti-air... combo counter to counter combo rush.
but you should be focused on attacking too... if you are just hanging about waiting to be attacked, you will lose everytime. Guaranteed.
It's hard to get an even game anymore in anything higher than 2v2...
Me? I LOVE to see somebody put walls up... 1) it means they aren't spreading which leaves more room for me and 2) it means they wont have as many anti-air and turrets in their base.
Personally, I DON'T build turrets or anti-air. I just plop fortresses all over my base. It is unusual for me to have less than 6 fortresses by the end of any large game that I didn't get slaughtered in early on.
Kind of rambly, I know.. but that is my two cents worth...