We played a three-way battle with 17,000 points to a side (see pics here). There wasn't much fluff to it - just a 'king of the hill' approach.
Army must control three objectives, including the center objective. We each had two in our backfield.
Orks vs AMTL vs Biel Tan on an irregular 13x13 board. The result was Biel Tan ate the other two up
but the truth of the matter was that the board set up and terrain had huge implications for the win. My recommendations are:
1. Make sure you have a couple of days to play. It is a shame to set all that up just to clean it up the same day. It took us 3 hours to set up the armies and 10 hours to play two turns.
2. Measure distances to objectives carefully, especially if using an irregular shaped board.
3. Make sure that the set-up areas are roughly the same distance from one another. AMTL were about 10 inches closer to Orks than to Biel Tan, which pushed them quickly into combat. Orks were distanced about 20 inches further from Biel Tan than from AMTL, but it wasn't so obvious on such a large board.
If you want a scenario with fluff, have 2vs1. Who says it has to be balanced? Give the sole player some advantages like two Supreme Commander re-rolls (not to be used consecutively) or some extra points, better terrain, last set-up, easier victory conditions, whatever...
Or a free for all where one army occupies a facility that the other two desire. Similar to a 2vs1 but they two armies are under no obligation to help each other.
Convoy across the battlefield. Have one army moving to support another army, but they must also escort non-military vehicles to their ally (on the opposite side of the board) safely in order to succeed. This forces the supported ally to attempt to draw as much fire as possible from the convoy to get their goods, but still have enough forces to protect their lines. The sole army has to balance between the juicy target and the opponent who is slamming them as hard as possible.
Just some ideas off the top of my head. PLEASE TAKE PICS!!!
and a huge IG army which i will call: