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I disagree with what Flunker said. Maybe some people like to play that way, but it's up to you. Personally, even for 3v3, I like big fields with deep wooded areas, open areas, and light woods. For a bunker, it depends on what you want. Some people would use a shed with windows/gun slits around the side. If you want to make it yourself, try making an octagon out of wood and taking one of the sides out. Make it about 1-2 feet tall, and dig a 2 foot hole behind it. This will give you plenty of space to hide behind. You could also put two sticks in the ground about 2 inches away from each other. 2 feet away, do the same set up. Then stick some more sticks in the hole it creates. You now have a wall without any nails, ropes, etc.
I would check out some sites that have layouts of fields and go from there. Depending on how long you want the games to go, and how many people you have, the size of the field should vary. You don't want a 3 on 3 game on a huge field, i.e. 200 yds x 100 yds, otherwise the game will take forever.
As far as layouts go, try to make a big bunker in the middle, for this you can maybe find some old tires and build them up to make like an H design and then around the rest of the field, use the terrain to your advantage and create cool obstacles that provide some good angles to other bunkers you have placed.
One other comment, would be to keep the bunkers symmetrical so one team doesn't have an advantage over the other.
If you've got a forested property with plenty of vareity in terms of geography and tree density... your designing stage is pretty much done. The land you have in this case will be fine at first and you can make modifications or add things as they come up.
If you're on a flat piece of land with no variety, whether it's treed or not... you're going to have to invest some time building objects for cover and obstacles. These are usually simple plywood shapes supported by 2*4 frames. You can also set up nets covered in artificial or real foliage as walls or blinds.
In the end it's a balance of creativity of the field owner and fun for the players... and that's something nobody can teach you.