In addition to the good info others have given, you can send off samples for analysis to be precise about it. Different types of plant have slightly different nutrition too. While it depends on the pasture, soil, fertiliser, etc, etc, it is likely your horses will get most of the nutrition they require from the grass, but will be imbalanced & benefit from supplementing certain nutrients. FeedXL.com is one great resource for working out what/how much exactly. Especially if you get the pasture tested, you will know what needs to be supplemented.
They also have different sugar/NSC levels, which is IMO a more important consideration with horses. Rye grass for eg is very high sugar(developed for cattle fattening & milk production), so is not great for horses, especially those already 'laminitis prone'. I would be concerned about keeping easy keepers/lami-prone horses on this type of pasture without restriction, especially if fertilised. Orchard grass is apparently(I'm not from there) a lot lower NSC, more of a native grass. Have no idea about Kentucky & think fescue is a mid-range type. Fertilising grass can also raise the NSC, sunlight does, stress/overgrazing does, among a number of other factors, and hay retains the sugar that's in the grass when cut.
So... it depends, but on average, 3 horses on 7.5 well managed acres could work well without having to actually feed them extra. Horses need approximately 2.5% bwt daily in dry forage, so more than that in fresh, juicy grass(feedXL also has a good estimator for how much grazing), depends on how lush the grass, etc. So it depends as to whether/when/how much you may have to feed out hay. If pasture is too 'good', you will likely need to restrict grazing, esp if your 'prone to founder' horse is overweight/IR, especially with the rye grass.
Remember, hay loses many nutrients in processing, but it doesn't lose sugars, so if feeding 'improved' hay such as rye grass, may be best to soak it first to leach out sugars before feeding, esp to your lami-prone horse. You may consider feeding your horses soaked hay rather than allowing them free access to the pasture in Spring, on summer afternoons & other times the pasture is particularly rich in sugar.
Legumes, as in alfalfa, aren't necessarily bad for horses at risk of laminitis. Alfalfa is generally lower in sugar than most 'improved' grass varieties, so can actually be safer. Legumes are also relatively high in magnesium, which is generally lacking in the diet & very beneficial. *Although Mg is lost through many modern agricultural practices & Ca may be too high. Alfalfa is high calorie though, so may not be best for 'easy keepers', of which tends to often go along with 'lami-prone' because the cause is commonly like type 2 diabetes.
You can learn more about pasture & laminitis risks at Katy Watts | Safergrass.org
& also at Home
I would also urge you to look into providing extra magnesium in their diets. Magnesium for Horses | Natural Health for Equines
is one of many sources of info online on magnesium studies.