Why with a "
tri-axle trailer" ???? makes no difference to the tow barge

... in fact it makes it harder to control ... three axles are gonna want to go straight on, far more than a twin will

.... the load is on the trailer, two or three axles, it's still 3.5t, as far as towing it is concerned?
As I've already said, most couplings are only rated at 100kgs
MAX nose weight, so any more 'load carrying' ability added to a tow bar is superfluous, unless you want to put a m/cycle rack on it .
Even an old ring/eye coupling is still only rated at 100kgs ... any more and the over-run mechanism, for the smooth operation of the brakes, will not work properly
Think you may be thinking of the old
'green to go' indicator on some couplings, which told you the coupling was correctly secured and locked to the ball .... was now't to do with nose weight