Martin Crawford said:
On a different matter, I had a hugh fishtail with my 3 ton load on the way down the other side of that steep hill.
It frightened the be jasus out of me and other road users too.
Been there - with two horses in a trailer behind an 88" Land Rover

Its amazing how much road space I had by the time I stopped on the hard shoulder
Do you have the long wheelbase KZJ95 ??, more stable with a heavy trailer than the short wheelbase KZJ90 models.
3 tons is a bit OTT for the Colorado so you have to be careful - the LWB only weighs 2 tons (empty), so downhill its easy for the trailer to take over.
Is the trailer level when hitched-up ??
Is the nose weight in the right ballpark - it has to be positive and around 100 Kilos.
I'm assuming its a twin axle - are the tyre pressures right - and all the same ??
Does the trailer have high sides and were you in a cross-wind ??
Was the load centred between the wheels ??
Are the brakes adjusted properly so that none are binding and all come on evenly ??
Does it have a hydraulically-damped hitch (the old sprung ones can snatch and cause problems) ??
A heavy trailer with a short drawbar will be more prove to weaving than one with a long drawbar. Its my Rice trailer with 6X16" wheels and a short drawbar that weaves with me. The Ifor Williams with 13" wheels and a long drawbar has never given me any concerns (but I am paranoid about keeping everything working properly).
Even with everything within spec, you can get caught out by rough road surfaces, camber, cross-winds, making steering adjustments, having to slow on a down-grade

The joys of towing.
Bob.