I don't think that would be a good idea, because whichever bozos are milking would milk all BUT that field, then peeps would join, be stuck in that horrible field, and leave in disgust. No, there must be another way.
I put a request in for a 2-sided map, a way of having a map where you are not FORCED to have 3 sides, to HTC a while back. Not sure how hard that is to code, nor am I sure if they'll ever do it, but it might ease the problem a little.
[EDIT] Is there a way to make a base "un capturable"? Meaning say you've got a map with 30 axis fields, right? Make the 2 rearmost fields "fallback" fields, that can never be captured, can never be porked... Or hell, maybe just make the rear most bases up the wazoo with flak so that NO enemy EVER will be able to take them. That way the one side being porked will ALWAYS have some place to launch from to re-take their fields.
I think that is a better idea than the original one (surrounding 1 with 3rd team bases)