What do you guys see as the best strategy when/if you are attacked by 3 player at the same time?
Should one be extremely defensive and duke it out at your "home castle" or should you leave your castle (more or less) defenceless and in an attempt to take out one of your attackers?

If there are 3 against you then you are doomed . I would try the following in order:

1. Most other players would probably help you if you are being attacked against such odds, so ask all other players in the game
2. Some players will break off an attack to avoid attacking you so unfairly, so try diplomacy.
3. If 1. and 2. don't work, I would select your weakest opponent (spy on them if necessary) and make a rush for their castle. You won't last long against 3 players unless they are much weaker than you, so you may as well go out fighting.

It might be hard and it would depend on how high your tech is.

Place outpost for a good view of your territory.
Set up towers in position to provide cover into a couple areas of your land, then place ranged units within.
(note if you don't have towers outpost work but they only give one extra hex in range)

Keep your melee units in a position where they can charge out just short of the max range of your towers. When an enemy comes in range hit him with the tower units, send your melee units out to attack. After there attack drop your melee unit back one hex. (keeps your enemy from being able to hit you with more than 100% atk)

It also helps to have extra archers an melee units on the ground to cover any gaps as you pull back. If you work it just right you should be able to hold out.

yeah take it from someone who face that problem in his last game and finish the game in first place the cat has made the best suggestions i would suggest u get an allie and get him after some one who wont break the fight then it should be 2vs 3 or 2 vs 2, limit ur causalites and and move with caution use any strategy that works best avoid being over taken

yeah i am currently being attacked by three people in my game slow 27. but i believe that is because there is about 4 turns left and i am in first. by a large margin. so i just beefed up my castle defences and am prepared to duke it out for the last few turns.

i've been surviving an off and on 3 v 1 for 29 turns in skirm 54. they (dumbly) hit me just one or two at a time only though. i just hit them with everything i got and they go away for a while at least. attrition is starting to get to me though

Good ideas overall but the problem is that we are only 4 players left in the game...and the rest of them are allies.
But I will go out with a (gang)BANG!

Do not lock your self up, becouse then you loose your resources, and then its downhill.
Plan your defenses and attacks, remember and use the ZOC to your advantage, and nail them with range (if they get stuck on ZOC among themselves much better).
When you attack, do it in mass against single stacks and get rid of complete stacks, not just weak them (unless you want a very low count stack for ZOC control pourposes with the other 2 enemies, then it is very good)
And do not let go, fight until they are drained.

A slight update on my progress.

I rushed my closest enemys castle and will soon take it over.
The other two are slowly making their way towards my "home" castle but I will (by the time they arrive) have had time to rebuild my army.
I just might have a slight chance to survive this game.
Thanks for all your tips and good ideas...

if you know they are moveing towards you,,,then send out troops as far as you can and keep the battle as far away from your castle as possible,,((even if you lose the battle you will slow down their advance giveing you time to produce alot of troops))

3 vs 1? call your 4 allies.

You weren't paying attention pinafore. He has no allies! Next time he'll be more social

Endgame. Very messy
Unfortuantely, this is where I tend to lose it in campaigns... :\