Saw a Ford cargo 814 with 3 horses today and just thought it must have been over weight. It had quite large living and the horses (not ponies!) looked a bit squashed!
It could be possible in theory. My lorry (though about to get a new chassis) is an 811. The key though is the box - it's aluminium, so lighter then glasonite, which most are made of. Without horses mine weights 5.5 ton, (and it's got a mega long 24ft box!) so 3 normal weight horses would be ok without filling a water tank etc.
However bear in mind there is also supposed to be even weight over the two axles. Engine and cab/living at the front could balance out the three horses at the back, so it might have been ok.
To be honest there are so many 7.5 ton lorrys out there that travel overloaded, it's a wonder the Police & Ministry aren't out pulling them over & having them weighed.
Most overweight horseboxes appear to be conversions as they are invariably heavier to start with. If the body has been purpose built & is dropped onto a chassis then it should be within weight providing the living hasn't been over done.
I have a 7.5 ton purpose built lorry with fairly simple living & I have a payload of 2 tons so I should be able to carry 3 horses although I normally only carry 2.