Hi sjseng. I'm new to the forum, however not too new to this subject. I've driven tractor trailer units while full of logs in the rockies for a number of years, and had to "chain up and chain off" too many times to count. Most of the time on the top of a logging road in the middle of no where.
Others stated that chaining up the right rear outer, and front left outer, were right. However that only applies to a Tandem axel with a power divider installed. (which I'm sure does not pertain to your situation, as well as not pertaining to your question)
On your trailer, if you only have 1 set of chains for the 3 axles, then you would be a bit more sensible by chaining up the front axle on both sides. The reasoning behind this is that if you think about coming to a stop, even on dry pavement, you will notice that gravity as well as weight come into play. Whereas the trailer, as well as tow vehicle, have a tendency to lean forward onto their front wheels, thus leaving the rear wheels not having the same amount of stopping power as the front ones. Thus, by putting the chains on the forward axle, you should be able to control the Trailer better than having them on the rear axles. You shouldn't be going very fast at the time, so control of both units should be fairly easy. (try not to go more than 10 mph while chained up) Once you feel confident enough to carry on without the chains, then remove them, then increase your speed.
Hopefully that helped you and I sincerly hope that I'm not confusing you.
Good Luck and if you "have to" go on, then please take it easy.