Dogtrousers
Kilometre nibbler
I wouldn't assume that grass verges are OK for horses. I don't know much about horses but I do know they can be fussy and difficult to handle, so it may not be safe to take those particular horses onto those particular verges.I'm sure it's fine, it's when they know there is traffic behind them it becomes an issue, you get people getting impatient and doing stupid stuff.
I have followed horses for miles in a car on a back road, 3 abreast with plenty of places to move over or grass verges to allow traffic to pass and they didn't then they get a close pass from an impatient driver, in this case I didn't care as I wasn't in a rush to get anywhere, but others were honking and shouting.
Edit: I'm not excusing a close pass, I'm just saying sometimes it's from frustration and impatience.
I've never seen horses 3 abreast on the road. Only ever 2. You must have got very unlucky in that instance. Anyway, the highway code for horses is the same as for cyclists - never ride more than two abreast.