The simplest model of rotational crossbreeding is the two-way cross where two different breeds are crossed. The next generation is called F1 and if the offspring from this cross is mated back to one of the original breeds, this is called a backcross.
The highest level of heterosis is always in the first generation and the level decreases in following generations. When F1 cows are backcrossed, in generation F2 the heterosis is halved compared to the level in the F1.
The heterosis rises again in the F3 generation but then levels to 67% in few generations.
The three-breed crossing can be seen as the optimal crossbreeding system as the heterosis stays higher than in two-way crossbreeding. In three-way crossbreeding the first generation is also called F1, but instead of starting the backcrossing with the cows, the cows are mated to a third breed.
The heterosis stays at 100% for the first two generations, but then drops when the first backcrossing is made to one of the original breeds in F3 generation. After few generations the heterosis level steadies at 86%.
You keep the desired traits in dairy production, but as an added benefit the crossbred offspring will have improved fertility and health traits.
With four-way crossbreeding even higher heterosis can be achieved but maintaining the correct rotation gets more complicated. It's harder to find breeds that complement each other well and are unrelated populations. Therefore, the four-way crossbreeding is not recommended.