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When winning a match, you most likely already control two bases. At the end of the game check out the third base and see if there's a fight going on. Chances are good, that when it's going to be a certain defeat, the opposing faction will quit fighting after they died and peacefuly wait for the game to end, thus giving you a great chance to cap the missing third base in the last seconds of the match.
Try it out, you'll be surprised how well that works out.

I haven't been keeping very good track of this, but I feel like I've done it already twice. Yet I'm still at 1/10 on achievement list.

Does the base have to be fully capped and controlled for this to count? Or can you steal it at the last second and it count towards achievement just because you tagged it?

You have to actually control all 3 bases. If you capture a flag just before the battle ends, you don't control the base yet, and therefor will not get it counting towards your achievement.