In answer to your question, Charlie, yes a tractor can tow a three horse float. Some are capable of towing 20 - 30 ton trailers, quite often seen in Ireland and Europe where they are used in preference to prime mover because of their agility in narrow roads. However, as Cobba said, you would need to have electric brakes on the trailer and the companion unit fitted to the tractor.
The tractor brake unit could be fitted to something as small as a grey Fergie, but it would not be legal to tow that much weight. You would need quite a heavy tractor for the job.