I've had horses off and on for years. The most that I've had at once was five and I currently own two.
None of that makes me an expert by any stretch, but........ I have learned from experience what works well for me and mine.
To my way of thinking, hay is basicly just dried grass.
Having said that though, I do know that the two do have different properties and are not fully interchangable as to diet. So what I do is........feed a little hay daily to go along with the pasture-grass they are getting. Doing that seems to aid the transition from grass to hay, as the grass dies off during winter.
One doesn't want to make abrupt diet changes, even though the cost-factor of feeding hay(and/or feed) all year around may tempt one to do so.
And beings how you limit yours to 3 hours of grazing a day, I doubt that adding a little hay each day will make them fat. Besides, even if it does, a bit of added exercise will cure that.
I'm also a big believer in vitamin and mineral suppliments.
Mine get theirs through the feed-mix I use, which is divided-up into 2 feedings a day.
Given the fact that plant-eaters(as opposed to canavores) must get all their minerals from the plants they eat....and...given the fact that soil gets depleated of it's mineral content rather quickly....and....given the fact that most fertilizers contain little or no mineral-boosters.... I see no way that pasture grass(or hay) alone can provide anything close to a balanced diet.
On the other hand, I'm in S/E Texas and you are in Nicaragua.
You may have much better grass than we do and/or other factors involved which I would not be aware of.
Hope this helps.