You will need all the original files, then you can do a conversion to AH3 from the AH3 terrain editor. If you don't have them, it's not that hard to create a small terrain for use in the Player tab area. It would be a good way for you to learn terrain building and you can contact Esayscor to get a link to his PDF manual. You can make very tiny terrains for use in the Player tab area as long as you meet the basic upload requirements of the HOST uploader.
5-10K canyon with elevated takeoffs approximately 10 miles apart. <--- This is very simple if you only want a 10k high island about 15 miles in diameter. Set the elevation tool to 6mile dia, and 10000 elevation. Then click in a 15mile diameter blob of land with your mouse. Then make your alt brush 3mile dia and 1000 elevation, and cut your canyons with your mouse. Add 3 airfields 10-13 miles apart, make them one of each country. Add an HQ for each country off in a corner and you have a terrain that should pass the HOST upload req. It won't be pretty but, you guys don't really care about eyecandy....do you....
There will be a few house keeping chores Easyscor will get you through like creating a clipboard map so you can see the terrain and running the error checker and leveling square objects to the terrain. But, something that small should take about a few days to a week. Versus the monster I've attached screen shots from. Oh!! get Easyscor to help you place the map room on the airfield so you don't have to deal with towns since all you want is to duel and furball. And if you only want two airfields, stick the third country airfield with it's HQ on some tiny island out of the way. This way if it's only a single big island, you don't have to create a blue print art file in 1:1 8pixel=1mile scale to design it to scale, then keep track of your monstrosity's production process. A 15mile dia land blob is finger painting with the mouse since you have a 1x1mile, 20x20mile grid to guide you. Also make sure the project name is in small letters and no more than 8 letters or, you won't be able to open the terrain to get into a tower. Now if you do goof with the naming convention, I'll tell you how I fixed that one after I thought Hitech had rolled out a bug in the patch I updated with that day.
Otherwise it's just mind numbingly time consuming if you are creating anything larger than the central furball island on the current terrain I'm finishing up.

