Good luck,
Hitech didn't like the idea of my making the three small vBases in the tank caldera on BowlMA uncapturable and he made me change that. That is why I used those tiny vBases with indestructible vehicle hangers and a spawn from the mainland to facilitate a quick recapture by a lone M3 that can easily de-ack the two auto guns. So with Oceania I didn't even bother to test Hitech's patience with the subject. That is why there is a GV spawn from the mainland to support an air assault with a troop M3 and tanks. It still amazes me no one sees why I placed that spawn from the main island there.
As for 3 miles, any closer and it turns the M3 into a super asset becasue of it's speed and players do swamp towns with M3 hoards. Placing the town farther away favors the attackers while slowing down the defenders time to the fight. Placing spawns closer ends up with abusive greifing on every MA terrain that has a really short spawn to the town and field. Fortunately that is only a few across all the converted AH2 terrains in rotation. I spent about 3 weeks with a stop watch driving and flying to come up with 3 miles so both the defenders and attackers had the same chance to win in the race for the town flag. It works well with bridges and a river one mile from the spawn with map room on the field setups. After three MA terrains back to back over two years, it seems to work for Hitech's customers keeping them engaged in activity versus driving forever or getting greifed by disparities in transit time.
If you make the three feilds uncapturable, then you end up with a furball lake scenario and people not "playing MA". The MA is about more than devolving into the DA furball lake to Hitech I will venture.