I like the way you're thinking but when three bulls are turned in with 90 breeding age heifers the equation isn't, 3 bulls = 30 each.
The equation is 3 bulls = 90 each. There are going to be heifers that are covered by all three bulls if they are doing their job. It really depends on several factors, like how large an area they are in and how good or sparse their forage is. If they are all in one bunch on lush pasture each of the bulls is going to be covering a larger percentage of the heifers. If the heifers are spread from here to kingdom come then the bulls will have to travel more and will cover a lower percentage of heifers. They'd also be more likely to be losing weight.
If the bulls are gaining weight it's most likely they are on good pasture and they settled the biggest percentage of the heifers in the first estrus... and then had light work for the rest of the 45 days, only breeding the few that didn't breed earlier.
As far as giving them a rest period... why? If they are gaining weight, or at least not losing weight, then they aren't working very hard.